孕早期颈项透明层超声联合孕中期彩色多普勒超声可提高产前胎儿畸形检出率

目的探究孕早期颈项透明层（NT）超声联合孕中期彩色多普勒超声在产前胎儿畸形筛查中的应用价值。方法选择2017年8月~2019年12月在我院行胎儿畸形筛查的2417例孕妇作为研究对象,所有孕妇于孕11~14周行NT超声检查,并于孕22~28周行二维、四维彩超检查。以引产或分娩结果为“金标准”,比较畸形胎儿、正常胎儿的NT值及NT异常率,对比NT超声检查、二维联合四维彩超检查结果,并就NT超声、二维及四维彩超及二者联合在诊断胎儿畸形中的诊断率进行比较。结果2417例孕妇最终确诊异常胎儿88例,发病率为3.64%。畸形胎儿的NT值明显高于正常胎儿,NT异常率（93.18%）明显高于正常胎儿（1.33%）,差异比较有统计学意义（P < 0.05）。NT超声检查共检出异常胎儿82例,诊断准确率为93.18%;二维联合四维彩超共检出异常胎儿85例,诊断准确率为96.59%。二维联合四维彩超的诊断准确率略高于NT超声检查,差异无统计学意义（P>0.05）。NT超声联合二维、四维彩超在诊断胎儿畸形中的敏感度、特异度和准确度分别为100.00%、99.57%、99.59%,高于NT超声、二维和四维彩超（分别为93.18%、98.67%、98.47%和96.59%、99.06%、98.97%）,组间比较差异有统计学意义（P < 0.05）。结论孕早期NT超声及孕中期彩色多普勒超声在产前筛查胎儿畸形上各具有优势,均具有较好的诊断价值。二者联合应用能够进一步提升胎儿畸形的检出率,对尽早终止胎儿畸形孕妇继续妊娠,减少畸形胎儿出生具有重要意义。      

Doppler ultrasonography in early pregnancy does not predict adverse pregnancy outcome.

A transvaginal color and pulsed Doppler study was performed on 330 pregnancies at 7-16 weeks of gestation. Blood flow velocity waveforms were recorded from the main uterine arteries and their branches (arcuate and trophoblastic vessels) and from the umbilical arteries. The systolic/diastolic velocity ratio and the pulsatility index were calculated as indices of vascular resistance, respectively, at uterine and umbilical levels. A total of 282 patients had an uneventful pregnancy outcome, 19 developed an early pregnancy failure (missed abortion, n = 8; anembryonic pregnancy, n = 11) and 29 developed later complications such us hypertension (n = 10), fetal growth retardation (n = 13) or both n = 6). In normal pregnancies, Doppler-measured vascular resistances significantly decreased with advancing gestation at the level of both uterine and umbilical circulations. When the Doppler indices of the patients with early pregnancy failure or with later complications were compared to those of normal pregnancies, no evident differences were found in either of the vascular districts considered. However, in the patients who developed hypertension and/or fetal growth retardation, abnormal velocity waveforms were frequently present in uterine and umbilical arteries during second- or third-trimester examinations. Our results suggest a limited clinical value of Doppler velocimetry in early pregnancy.     

超声筛查孕11~13+6周胎儿颅脑畸形的现状及意义

随着影像技术的发展,超声医师技能的提升、经验的积累及产前超声对胎儿中枢神经系统(CNS)的正常发育特征及异常表现的深入研究,超声检出胎儿CNS畸形的时间越来越早。目前在孕11~13⁺⁶周,超声可以诊断的颅脑畸形有无脑畸形/露脑畸形、脑膜膨出/脑膜脑膨出、全前脑、脉络丛囊肿;同时还可提供CNS异常发育的线索。本文对超声在孕11~13⁺⁶周胎儿颅脑畸形筛查中的现状及意义进行综述。     

子宫动脉超声多普勒血流监测在妊娠期高血压疾病晚期中的应用

目的 探讨子宫动脉超声多普勒血流监测在妊娠期高血压疾病晚期中的应用价值.方法 超声检测68例妊娠期高血压疾病和47例正常妊娠孕妇(对照组)的子宫动脉血流动力学参数:收缩-舒张流速比(S/D)、阻力指数、搏动指数,比较两组子宫动脉各血流参数及妊娠结局的差异;同时将妊娠期高血压疾病阻力指数正常组与异常组的妊娠结局进行比较....     

Ultrasonographic characterization of the uterine artery in the nonestrus bitch

The canine species is often used as an experimental animal model in medicine. Doppler ultrasound of uterine arteries has important clinical implications in obstetrics and gynecology in women and animal species. To our knowledge, the ultrasonographic features of uterine arteries in the bitch have not been reported. Thus, an ultrasonographic technique to locate the uterine arteries in the bitch is described in this study. A total of 11 nonspayed nonestrus bitches underwent abdominal ultrasound examination. Pulsed-wave, color and power Doppler settings were used to detect the location and characterize the flow of the uterine arteries. Mean values for systolic peak velocity, diastolic peak velocity, diastolic reversal flow and resistive index were calculated. Uterine arteries had a high pulsatility flow in all cases. Future work is needed to fully assess the clinical importance of the uterine artery flow in the bitch, as well as to study possible waveform changes at different estrus phases and throughout gestation. (E-mail: fernandoxy@yahoo.com)     

Pregnancy outcome and fibrinolytic, endothelial and coagulation markers in women undergoing uterine artery Doppler screening at 23 weeks

Summary.  Background:  Pre-eclampsia (PET) and/or fetal growth restriction (FGR) remain a major cause of maternal and fetal morbidity and mortality. In pregnancy, fibrinolysis is controlled by the maternal endothelium and placenta, both of which are central to the pathogenesis of PET/FGR. Clinically, uterine artery Doppler screening at 23 weeks is used to predict PET/FGR. An abnormal uterine artery Doppler finding is defined as early diastolic bilateral uterine artery notching (BN) in the waveform. However, about 50% of mothers with BN do not develop PET/FGR. Objectives:  We investigated fibrinolytic changes and uterine artery Doppler findings in the second trimester, and related them to pregnancy outcome; in particular assessing whether fibrinolytic markers could discriminate between normal and abnormal outcome in mothers with BN. Patients/methods:  Plasma levels of tissue-type plasminogen activator (t-PA), plasminogen activator inhibitor-1 (PAI-1), plasminogen activator inhibitor-2 (PAI-2), plasmin-α₂ antiplasmin (PAP), D-dimers and markers of endothelial dysfunction were measured with Doppler ultrasound at 23 weeks. Results:  Those with BN had decreased PAP and D-dimer levels, and raised PAI-1 and thrombomodulin levels. Mothers with BN and PET/FGR had significantly increased t-PA levels and reduced PAI-2 levels. Conclusions:  BN at 23 weeks of gestation is associated with increased PAI-1 levels. Within the BN group, mothers who developed PET/FGR had increased t-PA levels and decreased PAI-2 levels, although there was no net change in fibrinolysis as measured by D-dimer levels. No single fibrinolytic marker is helpful in determining pregnancy outcome in those with BN, but t-PA and PAI-2 are worthy of study in a multifactorial algorithm.     

多普勒频谱分析胎儿宫内缺氧

目的综合分析子宫动脉、脐动脉及大脑中动脉多普勒指标(PI值)变化关系对评价胎儿缺氧的意义。方法运用多普勒技术检测90例因不良妊娠住院患者子宫动脉、胎儿脐动脉及大脑中动脉搏动指数(PI),回顾分析妊娠结局与各频谱参数变化的关系。结果子宫动脉异常(PI≥95th)伴脐动脉参数升高(PI>50th)和(或)大脑中动脉参数降低(PI<50th)者妊娠结局不良,且脐动脉参数升高至异常时(PI≥95th)围产儿病死率增高;子宫动脉正常时(PI<95th),脐动脉参数异常(PI≥95th)伴大脑中动脉参数降低者(PI<50th)妊娠结局不良;大脑中动脉血流参数异常者(PI≤5th)妊娠结局不良;单一子宫动脉或脐动脉血流参数异常者(PI≥95th)妊娠结局良好。结论综合分析子宫动脉、脐动脉及大脑中动脉频谱指标变化关系,可判定及评估胎儿缺氧情况、提示预后。     

Ultrasonic measurement of fetal blood velocity wave form as a secondary diagnostic test in screening for intrauterine growth retardation

Based on routine fetometry screening at 32 weeks of gestation, 80 out of 3226 singleton pregnancies were suspected of intrauterine growth retardation (IUGR) and 72 of them were subjected to repeated fetal blood flow measurements by Doppler ultrasound for evaluation of the fetal condition. The maximum blood velocity wave form recorded from the fetal descending aorta and umbilical artery was classified as normal or abnormal depending on the presence or absence of positive end-diastolic blood flow. Compared to the pregnancies with normal fetal blood flow, the group of 30 pregnancies with abnormal blood flow patterns had significantly more pregnancy complications and more operative deliveries for fetal distress. In the latter group, all newborns but one were small-for-gestational age and had low 1-min Apgar scores more frequently. Fetal blood flow measurements have a good capacity for predicting unfavorable fetal outcomes and can be recommended for clinical use. The combination of ultrasound screening and Doppler blood flow measurement has reduced the number of pregnancies requiring intensive surveillance.     

超声检查习惯性流产患者早孕期胎心率在妊娠结局中的预测价值

目的探讨超声检查习惯性流产（RSA）患者早孕期胎心率变化在妊娠结局预测中的作用。方法收集行超声检查的妊娠42~69 d的孕妇，其中，有RSA病史孕妇255例，无RSA病史孕妇201例。根据妊娠42~196 d（即6~28周）期间妊娠结局分为3组:RSA异常妊娠组（有RSA病史孕妇本次妊娠期间发生自然流产）；RSA正常妊娠组（有RSA病史孕妇本次妊娠至28周以上）；非RSA正常妊娠组（无RSA病史孕妇本次妊娠至28周以上）。孕妇经腹或经阴道超声测量孕囊大小及胚芽、胎儿头臀长，启用M取样并放大模式测量胎心率，连续测量3次，取均值，记录孕囊大小、头臀长及胎心率，追踪随访至妊娠28周，比较各组间资料。结果255例RSA孕妇中，83.0%的胚胎停育发生在55 d前，胎心率越低，胚胎停育发生率越高。超声检查与胚胎停育时间相隔越短，胎心率缓慢发生率越高。RSA正常妊娠组与非RSA正常妊娠组各孕龄平均胎心率的差异无统计学意义（P > 0.05）。RSA异常妊娠组与非RSA正常妊娠组孕49~62 d平均胎心率差异有统计学意义（P < 0.05），孕42~48 d和孕63~69 d平均胎心率之间差异无统计学意义（P > 0.05）。用胎心率为检验变量，妊娠结局为金标准，预测胎心率诊断的价值，曲线面积为0.831，结果具有统计学意义（P < 0.05）。结论超声检查发现RSA患者早孕期胎心率缓慢对不良妊娠结局有一定的预测价值。      

胎盘早剥的声像图表现及妊娠结局分析

目的 探讨胎盘早剥声像图特征及妊娠结局。方法 收集132例经产后诊断为胎盘早剥孕妇的产前超声检查资料,分析其声像图表现及妊娠并发症。结果 超声检出胎盘早剥48例,检出率36.36%（48/132）,漏诊率63.64%（84/132）。胎盘早剥声像图表现为胎盘增厚29例、胎盘实质回声异常16例、胎盘后方回声异常9例、胎盘边缘回声异常6例、胎盘胎儿面回声异常4例、宫壁与胎膜间回声异常2例以及羊水池内回声异常7例;主要并发症为剖宫产、子宫胎盘卒中、弥漫性血管内凝血、产后出血、早产、新生儿重度窒息及围生儿死亡,经超声检出胎盘早剥者以上并发症发生率均高于超声未检出者（P均<0.05）。结论 胎盘早剥产前超声检出率低;超声检出胎盘早剥者妊娠结局较差。     

Use of colour and spectral Doppler ultrasound examination in gynaecology

Objective: To review and sum up the published literature on gynecological Doppler ultrasound examination. Methods: Publications on gynecological Doppler ultrasound examination already known by the author, publications found in the bibliographic database Medline, and publications found in the reference lists of available studies were read, and relevant information was extracted and summarized. Results: Reference data representative of normal findings at transvaginal color and spectral Doppler ultrasound examination of the uterine and ovarian arteries have been established in healthy pre- and post-menopausal women and in normal early pregnancies. Blood flow velocities in the uterine and ovarian arteries change during the normal menstrual cycle and are very different in pre- and post-menopausal women. Lower blood flow velocities and higher pulsatility index (PI) values have been recorded in the ovarian arteries after the menopause. Uterine artery blood flow velocities increase and uterine artery PI values and resistance index (RI) values decrease with gestational age in the first trimester. There is not yet an established role of the gynecological Doppler ultrasound examination in clinical practice. It remains unclear whether the gynecological Doppler ultrasound examination contributes substantially to the clinical management of early pregnancy complications or infertility problems, to the differential diagnosis of pelvic masses or uterine pathology. Conclusions: Large prospective studies—preferably randomized controled trials—are needed to determine the clinical value of the gynecological Doppler ultrasound examination.     

子宫动脉频谱形态异常可预测系统性红斑狼疮患者的妊娠结局

目的探讨子宫动脉多普勒在预测系统性红斑狼疮（SLE)合并妊娠母胎结局中的应用价值。方法回顾性选取45例系统性红斑狼疮合并妊娠孕妇,分析两组患者的临床情况及新生儿结局,并根据子宫动脉多普勒频谱是否出现切迹、搏动指数（PI)值增高,将患者分为正常组（n=31)及异常组（n=14),比较两组间的差异。结果45例孕妇中,15例（33%)出现子痫前期、狼疮性肾炎及抗磷脂综合征,胎儿出现不良结局共9例（20%)。子宫动脉频谱正常组31例（69%),5例（11%)孕妇发生子痫前期,1例（2%)胎儿发生小于胎龄儿。子宫动脉频谱异常组14例（31%),10例（22%)孕妇出现并发症,包括子痫前期、狼疮性肾炎,抗磷脂抗体（+)及抗磷脂综合征;8例（18%)胎儿发生不良结局:包括小于胎龄儿、胎儿生长受限及胎儿丢失。3例（7%)妊娠期新发SLE患者,孕妇子宫动脉波形均出现异常（波形出现切迹、PI值增高),3例胎儿均于围生期死亡。子宫动脉波形异常组母体并发症及胎儿不良结局的发生率均高于波形正常组,组间差异有统计学意义（P < 0.01)。结论孕妇子宫动脉频谱形态异常是预测妊娠期SLE加重及胎儿发生不良结局的敏感指标。      

Hepatic venous Doppler waveforms: Changes in pregnancy

A total of 75 fasted healthy normal volunteers were examined during various stages of pregnancy. Examinations were performed in a supine position using an Acuson 128 ultrasound scanner with a 3.5 MHz transducer. Doppler interrogation of the middle hepatic vein was made during quiet respiration by one of three examiners. The Doppler waveforms were subsequently assessed blindly by one of the authors and categorized as normal, damped, or flat. Between 12 and 20 weeks, the majority of patients had normal hepatic pulsatility (64%) with 20% damped and 16% flat. Between 20 and 30 weeks there was a significant change (p > 0.001) with 68% being flat, 20% damped, and only 12% normal. In the last 10 weeks of pregnancy the changes were more marked: 80% were flat, 12% dampened, and 8% normal. There was a profound change in hepatic venous pulsatility during pregnancy. Hepatic waveforms changed from their normal pulsatile nature to become completely flat with increasing gestation. These changes were more frequent and more marked the further gestation progressed. © 1995 John Wiley & Sons, Inc.     

Ultrasound diagnosis of severe thrombotic placental damage in the second trimester: an observational study.

OBJECTIVES: To screen women with uteroplacental insufficiency between 18 and 26 weeks' gestation for sonographic evidence of destructive placental lesions, to observe the effect of low molecular-weight heparin (LMWH) in these cases, and to compare the outcome with similar but untreated controls. METHODS: We screened 180 women at high risk for placental damage using 16-week maternal serum screening (alpha-fetoprotein and human chorionic gonadotropin), placental shape and texture, and uterine artery Doppler waveforms at the 18-20-week level II examination. Serial gray-scale examinations of placental texture were performed at 22, 24 and 26 weeks. LMWH was offered to women with ultrasound evidence of destructive placental lesions in the absence of intrauterine growth restriction and/or pre-eclampsia. RESULTS: We prospectively identified six women (3.3%) with abnormal maternal serum screening and uterine artery Doppler in whom abnormal placental texture (echogenic cystic lesions) suggestive of destructive lesions in the placental parenchyma was found either at the 18-20-week ultrasound examination (n = 4), or by 26 weeks of gestation (n = 2). All six received LMWH and had live births (gestational age at delivery, 33-37 weeks; birth weight, 1000-3200 g). A further 14 women were referred with similar multiparameter evidence of placental damage at or after 26 weeks, outside the screening study. All had significant fetal growth restriction and were therefore not offered heparin. In 9/14 cases there was a perinatal death. Ischemic and/or thrombotic placental pathology was confirmed in each case, but no maternal thrombophilia disorders were identified in the 20 women. CONCLUSIONS: Integrated biochemical and ultrasound testing of placental function at 16-20 weeks of gestation, followed by serial placental gray-scale ultrasound, may be an effective method of identifying a subset of pregnancies at high risk of adverse pregnancy outcome due to destructive lesions in the placental parenchyma. This strategy of identifying thrombo-occlusive placental lesions before the development of pregnancy complications may prove useful in the design of trials to study the effectiveness of LMWH in the prevention of clinical complications resulting from thrombo-occlusive placental disease.     

Flow index evaluation of 3-D volume flow images: an in vivo and in vitro study

Three-dimensional (3-D) ultrasound imaging has improved evaluation of organ circulation and might contribute new information on maternal and fetal blood supply. Flow index (FI) of 3-D color images has been proposed as a measure of perfusion. The aim of this study was to evaluate whether the 3-D FI is a parameter of volume flow and flow velocity in a human vessel and in a flow phantom. A 1-cm-long strip of the uterine artery was recorded in 3-D power Doppler (3D-PD) mode in a cross-sectional study of 170 normal singleton pregnancies between 26 and 42 weeks' gestation. A fixed ultrasound system installation was used during the examination. The VOCAL software integrated in the ultrasound unit calculated vessel volume and FI. Reproducibility of the measurements was tested. The method was also tested on a commercially available flow phantom. Reproducibility measurements gave satisfactory results, both in terms of inter- and intraobserver variation. Unexpectedly, in normal pregnancy, the uterine artery FI decreased slightly with gestation. Uterine artery vessel volume increased, however, with gestational age. A poor correlation was found between the FI and both flow velocity and volume flow in the flow phantom. In conclusion, 3D-PD imaging can give impressive anatomical pictures of organ vascular tree. However, the new FI is poorly related to flow velocity or volume of flow.     

Corpus luteum blood flow in normal and abnormal early pregnancy: evaluation with transvaginal color and pulsed Doppler sonography.

One hundred and thirteen (66.5%) women in this study had a normal intrauterine pregnancy with ages ranging 6 to 12 weeks of gestation. Fifty-seven (33.5%) patients were admitted to the hospital owing to clinically suspected abnormal early pregnancy. Dilatation and curettage were done on all women and tissue sample sent to the pathologist for a final diagnostic. Diagnosis of ectopic pregnancy was made on laparoscopy. Both ovaries were examined carefully by color Doppler in sonography in all patients. Color flow was used as a guide for pulsed Doppler exploration. Corpus luteum blood flow was defined as random, usually semilunar in appearance, dispersed vessels with very low impedance to blood flow. The resistive index and pulsatility index were calculated. Overall detection rate of corpus luteum blood flow in normal pregnancies was higher for the left ovary (62.6%) than for the right ovary (37.4%) (P < 0.01). The mean resistive and pulsatility indices from corpus luteum blood flow were not influenced by gestational age in normal pregnancy. The overall mean value for for resistive index was 0.452 +/- 0.04 and for pulsatility index 0.636 +/- 0.09. The overall detection rate of corpus luteum in abnormal pregnancies also was higher for the left ovary (56.7%) than for the right ovary (43.4%) (P < 0.01). The mean resistive indices from corpus luteum blood flow in patients with missed abortion was higher than in women with normal pregnancy (P < 0.01). Both resistive and pulsatility indices were higher in patients with incomplete or threatened abortion in comparison with normal pregnancy (P < 0.01). No statistically significant difference was seen in the case of anembryonic, molar, or ectopic pregnancy.     

Uterine artery Doppler at 11-14 weeks of gestation in chromosomally abnormal fetuses.

OBJECTIVE: To determine whether the major chromosomal abnormalities are associated with impaired placentation in the first trimester of pregnancy. METHODS: This was a prospective study of 692 singleton pregnancies undergoing fetal karyotyping at 11-14 weeks of gestation. Uterine artery Doppler was carried out and the mean pulsatility index was calculated just before chorionic villus sampling. RESULTS: The fetal karyotype was normal in 613 pregnancies and abnormal in 79, including 39 cases of trisomy 21, 11 of trisomy 18, 11 of trisomy 13, eight of Turner syndrome and 10 with other defects. There were no significant differences in the median value of uterine artery mean PI between any of the individual groups. Although in the combined group of trisomy 18, trisomy 13 and Turner syndrome fetuses, the median pulsatility index (1.60) was significantly higher than in the chromosomally normal group (median pulsatility index, 1.51; P = 0.021), in the majority of abnormal fetuses (24 of 30) mean pulsatility index was below the 95th centile of the normal group (mean pulsatility index, 2.34). There was no significant association between uterine artery mean pulsatility index and fetal nuchal translucency thickness or fetal growth deficit. CONCLUSIONS: The high intrauterine lethality and fetal growth restriction associated with the major chromosomal abnormalities are unlikely to be the consequence of impaired placentation in the first trimester of pregnancy.     

Transvaginal 3-d power Doppler ultrasound evaluation of the fetal brain at 10-13 weeks' gestation

The objective of this study was to measure the fetal brain volume (FBV) and vascularization and blood flow using transvaginal 3-D power Doppler (3DPD) ultrasound late in the first trimester of pregnancy. 3DPD ultrasound examinations with the VOCAL imaging analysis program were performed on 36 normal fetuses from 10-13 weeks' gestation. FBV and 3DPD indices related to the fetal brain vascularization (vascularization index [VI], flow index [FI] and vascularization flow index [VFI]) were calculated in each fetus. Intra- and interclass correlation coefficients and intra- and interobserver agreements of measurements were assessed. FBV was curvilinearly correlated well with the gestational age (R2 = 0.861, p < 0.0001). All 3-D power Doppler indices (VI, FI and VFI) showed no change at 10-13 weeks' gestation. FBV and all 3-D power Doppler indices (VI, FI and VFI) showed a correlation > 0.82, with good intra- and interobserver agreement. Our findings suggest that 3-D ultrasound is a superior means of evaluating the FBV in utero, and that 3-D power Doppler ultrasound histogram analysis may provide new information on the assessment of fetal brain perfusion.     

超声多普勒监测胎盘循环的临床应用

本文用超声多普勒对40例正常妊娠和34例高危妊娠孕妇的胎盘循环进行了动态监测.并与妊娠结局进行了比较分析,认为超声多普勒胎盘循环的监测对胎儿宫内生长发育有预测价值,并对预测胎儿宫内环境,指导产科临床处理均有重要意义。     

First-trimester uteroplacental and fetal blood flow velocity waveforms in normally developing fetuses: a longitudinal study.

Our objectives were to obtain a complete overview of uteroplacental and fetal hemodynamics early in pregnancy and to record flow velocity waveforms in the vitelline artery using color Doppler equipment. Flow velocity waveforms were recorded in 18 uneventful pregnancies between 6 and 16 weeks of gestation. Doppler recordings were made of the left and right uterine and spiral arteries from 6 weeks of gestation onwards. Recordings of c the umbilical artery, fetal aorta and fetal cerebral arteries started in weeks 7, 8 and 9, respectively. Where possible, the vitelline arteries were also recorded. The resistance and pulsatility indices and 95% confidence intervals for predictions were calculated.For the uterine and spiral arteries, characteristics of the waveforms were assessed. The resistance index of both uterine and spiral arteries gradually decreased with advancing pregnancy. For the umbilical artery, aorta and cerebral artery, the earliest possible Doppler recordings and characteristics of the waveforms were established. The pulsatility index of the umbilical arteries and fetal aorta showed a sharp decrease towards the 16th week. The pulsatility index of the fetal cerebral artery showed only a mild decrease towards week 16. In a longitudinal analysis, there were significant changes of the resistance and pulsatility indices for each patient. In the vitelline artery, recordings were possible in eight out of 18 fetuses, and only in weeks 7 and 8 of gestation. The pulsatility index showed a wide range and was not dependent on menstrual age.Knowledge of the normal flow velocity waveforms in the maternal uteroplacental and fetal circulations early in pregnancy may lead to a better understanding of physiological mechanisms.