Levels of lipoprotein(a) in normal and compromised pregnancy

OBJECTIVE: To study maternal lipoprotein(a) levels in normal pregnancy and in pregnancy with evidence of vascular disease in the maternal uteroplacental circulation defined by Doppler ultrasound study. SAMPLES: Maternal venous blood was collected from 75 normal pregnant women and 68 pregnant women with evidence of potential uteroplacental vascular disease identified by Doppler ultrasound study. METHODS: Plasma lipoprotein(a) levels in maternal blood were measured using an enzyme-liked immunosorbent assay method. MAIN OUTCOME MEASURES: Plasma lipoprotein(a) levels and pregnancy outcome were examined. RESULTS: None of the normal group had lipoprotein(a) levels greater than 30 mg/dl, a cutoff level which has been associated with increased risk of atherosclerosis. 28 of the 68 women with uteroplacental insufficiency had lipoprotein(a) levels greater than this cutoff level. In this group there was a statistically significant higher prevalence of preeclampsia in comparison with women with a normal lipoprotein(a) level (p < 0.001). The lipoprotein(a) level was significantly higher in severe (n = 13, median 60.5 mg/dl, P < 0.001] than in mild preeclampsia (n = 5, median 34 mg/dl). Those with high levels (> 30 mg/dl) exhibited significantly more adverse indices of fetal outcome. CONCLUSION: This study has demonstrated that high levels of lipoprotein(a) interfere with uteroplacental circulation and play a role in the pathophysiology of preeclampsia. Lipoprotein(a) concentrations are associated with the severity of the disease. We suggest that high levels of lipoprotein(a) might affect the placenta and fetus.     

Second-trimester uterine artery Doppler screening in unselected populations: a review

Objective: Doppler ultrasound provides a non-invasive method for the study of the uteroplacental circulation. In normal pregnancy, impedance to flow in the uterine arteries decreases with gestation, which may be the consequence of trophoblastic invasion of the spiral arteries and their conversion into low-resistance vessels. Pre-eclampsia and fetal growth restriction are associated with failure of trophoblastic invasion of spiral arteries, and Doppler studies, in these conditions, have shown that impedance to flow in the uterine arteries is increased. A series of screening studies involving assessment of impedance to flow in the uterine arteries have examined the potential value of Doppler in identifying pregnancies at risk of the complications of impaired placentation. This review examines the findings of Doppler studies in unselected populations. Methods: Searches of a computerized medical database were performed to identify relevant studies. Only those studies that provided sufficient data to allow calculation of the performance of the test were included in the analysis. Likelihood ratios were calculated for each study and are reported for pre-eclampsia, fetal growth restriction and perinatal death as well as for more severe forms of pre-eclampsia and fetal growth restriction. Results: The literature search identified 19 relevant studies, four of which were excluded from the further analysis. The main characteristics and results of the 15 remaining studies provided discrepant results, which may be the consequence of differences in Doppler technique for sampling, the definition of abnormal flow velocity waveform, differences in the populations examined, the gestational age at which women were studied and different criteria for the diagnosis of pre-eclampsia and fetal growth restriction. Nevertheless, the studies provided evidence that increased impedance to flow in the uterine arteries is associated with increased risk for subsequent development of pre-eclampsia, fetal growth restriction and perinatal death. In addition, women with normal impedance to flow in the uterine arteries constituted a group that have a low risk of developing obstetric complications related to uteroplacental insufficiency. Conclusions: The review suggests that increased impedance to flow in the uterine arteries in pregnancies attending for routine antenatal care identifies about 40% of those who subsequently develop pre-eclampsia and about 20% of those who develop fetal growth restriction. Following a positive test, the likelihood of these complications is increased by about 6 and 3.5 times, respectively.     

Contrast-enhanced ultrasonography of uteroplacental circulation does not evoke harmful CTG changes or perinatal events

OBJECTIVE: To determine whether an intravascular ultrasound contrast agent used in the examination of uteroplacental circulation in late pregnancy has any effect on fetal cardiotocographic (CTG) parameters or fetal well-being. METHODS: The uteroplacental circulation of 69 women with singleton third trimester pregnancies was examined by power Doppler with contrast agent (Levovist((R)); Schering AG, Berlin, Germany) enhancement. Computed CTG examination using Dawes/Redman criteria was recorded just before and after ultrasonographic examination in 25 subjects examined with contrast agent and in 15 control patients who were given only physiological saline injection during ultrasonography. Umbilical artery blood flow velocity waveform was examined in 15 subjects before and 5 min after maternal contrast agent injection. Obstetric outcome of all the 69 subjects was evaluated by birthweight, placental weight, fetal and neonatal distress, prenatal hemorrhages, cord arterial and venous blood pH and macroscopic placental examination. RESULTS: No apparent fetal or maternal harmful effects caused by the contrast-enhanced ultrasonography were observed. In CTG examination there was a statistically significant increase in the number of accelerations and fetal movements and in short-term variation after administration of the contrast agent in both subjects and controls. There were no significant changes in the umbilical artery blood flow velocity waveform after administration of the contrast agent. CONCLUSIONS: Contrast agent enhancement in the examination of uteroplacental circulation does not produce any harmful fetal or placental effects in late pregnancy. Ultrasonographic examination itself, obviously due to maternal stress reaction, is associated with a slight activation of fetal behavioral status.     

Antenatal fetal surveillance

PURPOSE OF REVIEW: Antenatal fetal surveillance is a field of increasing importance in modern obstetrics, especially as results in perinatal care have recently made dramatic progress. It is an evolving field, and it is no longer acceptable just to wait and see when problems arise in pregnancy. During the past few decades many studies have shown that antenatal surveillance in unselected populations is of little value. However, high-risk patients benefit from antenatal fetal surveillance, especially women with pregnancy problems associated with intrauterine growth restriction. RECENT FINDINGS: This review shows that modern antenatal fetal surveillance is based on fetal heart rate monitoring, ultrasound biometry and amniotic fluid assessment, Doppler blood flow studies of fetal and uteroplacental circulation, and an evaluation of biophysical fetal parameters. SUMMARY: Used in combination these methods lead to improvements in fetal morbidity and mortality. The aim of future research should be to minimize the risks of fetal morbidity and mortality further by the optimal timing of delivery. Better organization of healthcare systems may improve our ability to identify at-risk patients during pregnancy. There is potential to improve the specificity of fetal surveillance tests, e.g. better methods of biometry and amniotic volume estimation with three-dimensional ultrasound and measurements of subcutaneous tissue. Improved knowledge of fetal physiology can be gained from research on fetal circulation with Doppler studies. Computer analysis of the fetal heart rate can increase the specificity of that test, and artificial neural networks may enhance the ability to evaluate the optimal use of integrated testing.     

Doppler ultrasound of the uteroplacental circulation as a screening test for severe pre-eclampsia with intra-uterine growth retardation

Two hundred primiparae underwent continuous-wave Doppler investigation of the uteroplacental circulation at 18-20 weeks gestation as a possible screening test for hypertension in pregnancy. Seventy-five women with abnormal waveforms suggestive of high uteroplacental resistance were tested again at 24 weeks when 21 demonstrated a persistent abnormality. Only nine (43%) of these went on to have an uncomplicated pregnancy, as compared with 150 (84%) of the remainder. Seventeen (8.5%) of the women in the study developed a hypertensive disorder of pregnancy, five of whom had abnormal waveforms at 18-20 weeks and at 24 weeks. These five women had a more severe degree of hypertension with proteinuria or intra-uterine growth retardation, and two required clinical intervention before term. The remaining 12 women were delivered at term of average, or heavier than average babies. Doppler investigation of the uteroplacental circulation at 24 weeks may prove to be a sensitive screening test for later severe pre-eclampsia with intra-uterine growth retardation.     

Ultrasound of the placenta: a systematic approach. Part II: functional assessment (Doppler)

Doppler velocimetry is the ideal clinical tool to assess placental performance in high-risk pregnancies. It also has value in predicting later complications and outcome in pregnancies which appear uncomplicated. All three circulations (fetal, placental and maternal) may be interrogated by Doppler technology. In the following review, we present basic physics aspects of Doppler and discuss mainly Doppler investigation of the fetal-placental circulation (umbilical artery, intraplacental circulation) as well as the uterine arteries. The assessment of umbilical blood flow provides information on blood perfusion of the fetal-placental unit. The diastolic blood flow velocity component in umbilical artery increases with advancing gestation. In pregnancies complicated by placental dysfunction, there may be a reduction in the number of functional villi and/or small blood vessels with, as a result, increased impedance, reflected, mainly, by a decrease in end-diastolic velocity. When the resistance increases even more, there is no diastolic forward velocity (absent end-diastolic velocity). Further increase in the resistance causes reversed end-diastolic velocity, which is considered a late step in the cascade of events leading to intrauterine fetal demise. Doppler assessment of the umbilical arteries was found to improve outcome of high-risk pregnancies, and reduce hospital admissions. On the contrary, routine Doppler ultrasound in low risk or unselected populations does not seem to confer benefit on mother or newborn. Uterine artery Doppler is a useful test in predicting pregnancies at high risk of developing complications related to uteroplacental insufficiency. It identifies women who may benefit from increased antenatal surveillance or prophylactic therapy. Three-dimensional power Doppler sonography can provide new insights into placental pathophysiology.     

Uteroplacental Circulation Development: Doppler Assessment and Clinical Importance

In the first weeks of pregnancy, columns of endovascular cytotrophoblastic plugs develop in the lumen of spiral arteries. Morphologic data show that these plugs become loosened as soon as the end of the second month and the intervillous circulation of maternal blood is likely to be established progressively between the 8th and 12th weeks. The disorganization of the musculo-elastic layers of these vessels provokes a dramatic decrease in vascular tone in the uteroplacental circulation. These modifications appear to govern the establishment of a low-pressure blood flow in the placenta, and hence determine the quality of uteroplacental circulation and normal fetal growth. Placental bed biopsies in women with pre-eclampsia and in a proportion of pregnancies with intrauterine growth retardation have shown that these physiologic changes were absent in the myometrial segments of spiral arteries. Recently, colour Doppler was used to assess intervillous and spiral artery flow in early pregnancy, confirming in vivo free intervillous flow at 12 weeks and a progressive significant decrease in spiral artery resistance with advancing gestation during the first trimester. However, certain data at an earlier gestational age are still contradictory. Particularly, the exact nature of the contents of the intervillous space before 8 weeks, and whether or not this fluid can be considered maternal blood, remains controversial.     

Intravascular ultrasound contrast agent: an aid in imaging intervillous blood flow?

The aim of this study was to evaluate the applicability of an intravascular ultrasound contrast agent in examination of the uteroplacental circulation. Uteroplacental circulation in 25 singleton third trimester pregnancies was examined by power Doppler, first without and then with contrast agent enhancement (Levovist, Schering AG, Germany). Eight subjects had fetal growth retardation and 17 had normal-sized fetuses. The effect of the contrast agent was evaluated using computerized power Doppler signal intensity measurements. The Doppler signal intensity in the uteroplacental vessels increased within 25 sec of brachial venous injection of the contrast agent, yielding an effect which was recognizable up to 5-8 min later. There was clear enhancement of uteroplacental flow imaging after addition of the contrast agent in all of the examined pregnancies. The mean percentual intensity changes after administration of the contrast agent were 33 per cent (P<0.001) in the sub-placental myometrial area and 8 per cent (P<0.001) in the intraplacental area. No association was found between fetal growth retardation and intensity changes or absolute intensity parameters. In conclusion, intravascular ultrasound contrast agent facilitates imaging of the uteroplacental circulation. It may have a valuable role in determination of intervillous blood flow and in imaging blood perfusion in different regions of the placenta.     

Studies of uteroplacental and fetal hemodynamics by Doppler ultrasonography

The role of Doppler ultrasonographic techniques in examining blood velocity waveforms in the different fetal and uteroplacental arteries is evaluated, and the association between pathologic blood velocity waveforms and early signs of disease is reviewed. These areas are discussed with respect to umbilicoplacental circulation, uterine circulation, and fetal cerebral and peripheral hemodynamics. Also reviewed are the use of Doppler techniques to determine the effects of vasoactive agents on the uterine and fetal circulation, and the use of color Doppler imaging in the exact localization of the short and small uterine and fetal vessels.     

Fetal and maternal hemodynamic responses to exercise in pregnancy assessed by Doppler ultrasonography

It is common for women to undertake vigorous exercise in the late phase of pregnancy. This may have detrimental effects on the blood flow to the uterus and placenta or from the fetus to the placenta. Fifteen pregnant women with no obstetric or medical complications were subjected to a 5-minute exercise period. The maternal heart rate and blood pressure were elevated after exercise. The uteroplacental and umbilical circulations were assessed with Doppler ultrasonography. The ratio of the systolic/diastolic velocity in the uterine artery was elevated, which suggests that uteroplacental vascular resistance increased. The fetal heart rate was elevated after exercise, whereas the systolic/diastolic velocity ratio in the umbilical artery was unaltered. We conclude that moderate maternal exercise causes increased resistance to blood flow in the uterine circulation, whereas the umbilical circulation remains unaltered.     

Colour Doppler Imaging of the Uteroplacental Circulation in the Mid-Trimester: Features of the Uterine Artery Flow-Velocity Waveform that Predict Abnormal Pregnancy Outcome

EDITORIAL COMMENT: We accepted this paper for publication because it presents data on the use of colour Doppler imaging in a screening test in healthy nulliparas to assess whether abnormal pregnancy outcome can be identified. Unexpectedly there were 4 perinatal deaths in this series and table 4 indicates that in the only case in which the perinatal death was potentially avoidable the 24-week uterine Doppler studies were normal. There is mounting evidence that Doppler imaging of uteroplacental circulation is useful in the management of identified high-risk pregnancies but the value of the method as a screening test remains unproven. If larger studies confirm the reliability of this investigation to detect patients who are destined to develop preeclampsia then a possible application of this is identification of women who warrant prophylactic aspirin therapy since recent results show that aspirin therapy is not indicated routinely in nulliparas
Summary: The accuracy of raised Resistance Index (RI) and early diastolic notching (EDN) of mid-trimester uterine artery flow velocity waveforms in predicting abnormal pregnancy outcome was assessed in a prospective longitudinal study using colour Doppler imaging at 14, 18, and 24 weeks. In 106 healthy nulliparas with a singleton pregnancy studied; 70 had a normal pregnancy outcome, 34 had an abnormal pregnancy outcome, and 2 were lost to follow-up. EDN was more sensitive than raised RI at predicting subsequent abnormal pregnancy outcome but raised RI was more specific at 14 and 18 weeks. Sensitivities were highest for the pregnancies with the more severely abnormal outcomes. Uterine artery Doppler studies in the mid-trimester can predict abnormal pregnancy outcome. The presence of an early diastolic notch is an important feature when defining an abnormal or high resisitance uteroplacental circulation.     

Interaction between risk factors for fetal growth retardation associated with abnormal umbilical artery Doppler studies

BACKGROUND: The role of antenatal risk factors associated with the occurrence of fetal growth restriction complicated by abnormal umbilical artery Doppler studies has not yet been studied extensively. We evaluated the role and the interactions of antenatal antecedents of fetal growth restriction complicated by abnormal umbilical artery end-diastolic velocities. METHODS: We compared antenatal variables in 183 pregnancies complicated by fetal growth retardation and abnormal umbilical artery Doppler studies and 549 appropriately grown fetuses with normal end-diastolic velocity waveform in the umbilical artery. Logistic regression was used to evaluate the association between antenatal variables and fetal growth retardation and to test for interaction. RESULTS: In logistic models, increasing maternal age [odds ratio (OR) 1.06, 95% confidence interval (CI) 1.01-1.11], nulliparity (OR 2.2, 95% CI 1.37-3.5), smoking during pregnancy (OR 2.56, 95% CI 1.56-4.22), preeclampsia (OR 27.5, 95% CI 15.1-49.9), first-trimester hemorrhage (OR 2.25, 95% CI 1.32-3.82) and low (< 0.2 kg/week) weight gain in pregnancy (OR 3.48, 95% CI 1.71-3.05) were significantly associated with an increased risk of fetal growth restriction complicated by abnormal Doppler studies. These risk factors were also significantly correlated with the occurrence of absent/reversed end-diastolic blood flow in the umbilical artery. Maternal smoking during pregnancy interacted negatively with preeclampsia but positively with a low weight gain in pregnancy. CONCLUSIONS: The results of this study have shown that antenatal risk factors for intrauterine growth retardation (IUGR) complicated by abnormal Doppler studies are similar to those associated with the birth of a small-for-gestational-age infant. Preeclampsia, maternal smoking and low weight gain in pregnancy play a significant causal role in the origin of fetal growth restriction associated with abnormal uteroplacental blood flow.     

Farbkodierte Dopplersonographie — Normwerte des Widerstandsindex in Abhängigkeit vom Gestationsalter

The examination of fetal circulation by means of Doppler ultrasound has clinical potential. In order to use the indices that characterize the waveform, it is important to establish reference ranges throughout pregnancy and to determine the natural variations. This paper sets out to fulfill these requirements. The study was performed in 139 patients with singleton pregnancies between 29 and 41 weeks’ gestation. Patients with no previous obstetric complications and no apparent medical problems were approached at the time of their visit to the antenatal clinic. All examinations were performed by a single observer. A 3-MHz pulsed waved duplex Doppler ultrasound instrument was used (Quantum 2000; Siemens, Erlangen, Germany). Doppler blood flow velocity waveforms in the umbilical artery, fetal aorta and middle cerebral artery were recorded. For each artery, reference ranges and a standard curve were performed for the resistance index (RI). The RI of the umbilical and the middle cerebral artery showed a continuing fall during gestation, but the resistance index of the fetal descending aorta did not change during gestation. The finding that the RI changes as gestation progresses indicates that fetal circulation increases throughout pregnancy. This suggests a decreasing placental resistance during normal gestation. Study of fetal circulation may identify patients at risk of developing severe disorders, thus allowing possible preventive therapy. Knowledge of the reference ranges helps to discriminate between a normal situation and mild or severe forms of disease.     

Umbilical artery and uteroplacental velocimetry in pregnancies complicated by idiopathic low birthweight centile.

Umbilical artery and uteroplacental Doppler flow velocity waveforms were studied in 35 pregnancies complicated by idiopathic low birthweight centile. Thirty fetuses (86%) were below the 5th centile birthweight for gestation. Fifteen (43%) had an abnormal umbilical artery systolic to diastolic ratio. Uteroplacental waveforms were recorded in 29 of the 35 pregnancies and five (17%) showed an abnormal systolic to diastolic ratio. Abnormal uteroplacental systolic to diastolic ratios were associated with a significantly earlier gestational age at delivery and significantly more cesarean sections for fetal distress compared with normal uteroplacental systolic to diastolic ratios (p less than or equal to 0.01). The absence of umbilical artery end-diastolic frequencies was associated with a significantly earlier gestational age at delivery compared with the presence of umbilical artery end-diastolic frequencies (p less than 0.005). No differences in pregnancy outcome were observed comparing normal with abnormal amniotic fluid volume assessment. These findings suggest that Doppler studies may be useful in estimating the risk of adverse perinatal outcome for small for gestational age fetuses with no identifiable cause.     

Intrauterine growth retardation and fetal cardiac function

Intrauterine growth retardation is a pathology which is found in 3-10% of all pregnancies and it is associated with around 20-25% of all fetal intrauterine deaths and with long-term neurologic sequelae. It presents an increased risk of distress during labor and delivery and a greater risk of perinatal mortality. The causes of IUGR and the cardiac and venous Doppler in normal fetuses are analyzed, and the hemodynamic cardiac modifications in IUGR fetus are discussed. The fetal cardiac function in intrauterine growth retardation shows a redistribution of the fetal cardiac output, which tends to favor the left ventricle as the mechanism to compensate for the uteroplacental insufficiency. The Doppler velocity indices are modified as the fetal condition progressively deteriorates and they represent an important tool for the management of the complicated pregnancy.     

Uteroplacental circulation in patients with first-trimester threatened abortion

OBJECTIVE: To assess uteroplacental circulation in patients with first-trimester threatened abortion with a living embryo. DESIGN: Prospective, cross-sectional study. SETTING: Tertiary care university hospital. PATIENT(S): Forty-nine patients with first-trimester threatened abortion and a living embryo and 129 women with singleton, low-risk, normally developing first-trimester pregnancies recruited as controls. INTERVENTION(S): Transvaginal color Doppler ultrasound measurement of the peak systolic velocity and pulsatility index of the uterine arteries and the spiral arteries. MAIN OUTCOME MEASURE(S): Uteroplacental blood flow and pregnancy outcome. RESULT(S): There was a significant relation between gestational age and the peak systolic velocity and pulsatility index in the uterine arteries and between gestational age and the peak systolic velocity and pulsatility index in the spiral arteries in controls. There were no differences in any Doppler parameter assessed between the study group and the controls, even in those pregnancies that ended in spontaneous abortion. CONCLUSION(S): No apparent alteration occurs in the early uteroplacental circulation in patients with threatened abortion with a living embryo. The use of transvaginal color Doppler ultrasound is not helpful for predicting pregnancy outcome in these cases.     

The assessment of placental blood vessels by three-dimensional power Doppler ultrasound

OBJECTIVE: This study was performed in order to compare the performance of classical two dimensional (2D) and three dimensional (3D) ultrasound, both with power Doppler technology, in the visualization of the placental vascular network during ongoing pregnancy. METHODS: 15 pregnant volunteers in the third trimester of pregnancy were examined by 2D and 3D power Doppler ultrasound. The aim of the study was to follow the branching of the main stem vessel as far as possible distally in the placenta. In addition, we assessed the visualization rate of terminal parts of uteroplacental circulation, radial and spiral arteries. RESULTS: There was no difference in the visualization of primary placental stem vessels by 2D and 3D power Doppler. However, 3D power Doppler performs better distally, with statistically significant differences at the level of secondary stem (p = 0.03), and even more prominent differences at the level of tertiary stem vessels (p = 0.0008). There was no difference in the visualization rate of radial and spiral arteries (p > 0.05). CONCLUSION: We found 3D superior to 2D ultrasound with power Doppler technology in the determination of the distal vascular branches of the fetal placental blood vessels.     

The effect of dihydralazine on blood velocity in branches of the uterine artery in pregnancy induced hypertension

Dihydralazine is frequently used in severe pregnancy induced hypertension (PIH). Little is known about its effect on the human uteroplacental circulation. In this study, Doppler ultrasound recordings were made from branches of the uterine artery in 5 women with PIH and blood pressure (BP) greater than or equal to 150/100 who received either 7.5 or 10 mg dihydralazine, as repeated intravenous doses of 2.5 mg, before obtaining near-normal/normal BP values. The measurements started 5-10 min prior to the injections and continued as undisrupted as possible during injections and the subsequent 30 min. When the reduction in BP was obtained, the median blood velocity was reduced by 23% (range 10-29%). A/B ratio was calculated at the same time as an indicator of peripheral vascular resistance. The ratio increased, compared with pre-experiment values, in 3 subjects (5, 26 and 31%) but decreased in 2 (4 and 6%). Maternal tachycardia was noted in all but one woman. Continuous fetal heart rate (FHR) recording showed no signs of fetal distress. The uteroplacental circulation does not seem to benefit from the vasodilatory effects of dihydralazine, the response to the decreased perfusion pressure being a reduction in blood velocity and unchanged peripheral resistance.     

References ranges and sources of variation for indices of pulsed Doppler flow velocity waveforms from the uteroplacental and fetal circulation

Summary. Variation in indices used to characterize the maximum frequency outline of waveforms obtained from the uteroplacental and fetal circulations by duplex Doppler ultrasound was studied between observers, over a 24-h period and before and after a meal. No significant differences were found. Reference ranges were derived for three indices, the A/B ratio, the resistance index and the pulsatility index, for waveforms from the fetal descending aorta, umbilical artery, and the placental and maternal sides of the uteroplacental circulation. Indices from waveforms from the uteroplacental circulation demonstrated a fall from 16 to 24 weeks gestation with no change thereafter. The umbilical artery indices demonstrated a continuing fall with increasing gestation but the A/B ratio and resistance index from the fetal descending aorta did not change with gestation. The pulsatility index from the aorta demonstrated a gentle rise.     

Doppler Flow Velocity Waveform Analysis in Postdate Pregnancies

This study examines the efficiency of Doppler flow velocity waveform analysis in the evaluation of the postdate pregnancy. Seventy women whose pregnancies were at or beyond 41 weeks' gestation were assessed by calculating the systolic/diastolic ratios from umbilical and uteroplacental arteries. Waveform ratios from pregnancies associated with fetal compromise and abnormal neonatal outcome were similar to those from pregnancies in which the outcome was normal. These findings indicate that the pathophysiology of placental insufficiency in postdate pregnancies differs from that observed in cases of fetal growth retardation at earlier gestational ages. Doppler flow velocity waveform analysis is unlikely to be of benefit in the routine assessment of the postdate pregnancy.