Comparison of gestational age at birth based on last menstrual period and ultrasound during the first trimester

Reported last menstrual period (LMP) is commonly used to estimate gestational age (GA) but may be unreliable. Ultrasound in the first trimester is generally considered a highly accurate method of pregnancy dating. The authors compared first trimester report of LMP and first trimester ultrasound for estimating GA at birth and examined whether disagreement between estimates varied by maternal and infant characteristics. Analyses included 1867 singleton livebirths to women enrolled in a prospective pregnancy cohort. The authors computed the difference between LMP and ultrasound GA estimates (GA difference) and examined the proportion of births within categories of GA difference stratified by maternal and infant characteristics. The proportion of births classified as preterm, term and post‐term by pregnancy dating methods was also examined. LMP‐based estimates were 0.8 days (standard deviation = 8.0, median = 0) longer on average than ultrasound estimates. LMP classified more births as post‐term than ultrasound (4.0% vs. 0.7%). GA difference was greater among young women, non‐Hispanic Black and Hispanic women, women of non‐optimal body weight and mothers of low‐birthweight infants. Results indicate first trimester report of LMP reasonably approximates gestational age obtained from first trimester ultrasound, but the degree of discrepancy between estimates varies by important maternal characteristics.     

Methods of gestational age assessment influence the observed association between antiretroviral therapy exposure,preterm delivery,and small-for-gestational age infants: a prospective study in Cape Town,South Africa

PurposeHeterogeneous findings exist on antiretroviral therapy (ART) use in pregnancy and preterm delivery (PTD) or infants born small-for-gestational age (SGA). Whether reported differences may be explained by methods used to ascertain gestational age (GA) has not been explored.MethodsWe enrolled consecutive pregnant women attending a large primary care antenatal clinic in South Africa. Public-sector midwives assessed GA by last menstrual period (LMP) and symphysis-fundal height (SFH). Separately, if clinical GA was less than 24 weeks, ultrasound (US) was performed by a research sonographer blinded to midwife assessments. In analysis, the impact of measurement error on the association between HIV/ART status and birth outcome by GA method was assessed, and factors associated with clinical GA underestimation or overestimation identified.ResultsIn 1787 women included overall, estimated PTD incidence was 36% by LMP, 17% by SFH, and 11% by US. PTD risk was higher for HIV-infected than HIV-uninfected women using US-GA (adjusted odds ratio [aOR] 1.95; 95% CI 1.10–3.46); for LMP/SFH-GA, the associations were smaller and not significant. These findings persisted after adjustment for age, parity, height, and previous PTD. PTD risk did not vary by timing of ART initiation (before or during pregnancy) for any method. Elevated BMI and older age were associated with decreased risk of underestimation by both LMP and SFH; HIV status and obesity were associated with increased risk of overestimation by SFH. There were no differences in SGA incidence across GA methods.ConclusionsFindings for an association between HIV/ART and birth outcomes are substantially influenced by GA assessment method. With growing public health interest in this association, future research efforts should seek to standardize optimal measures of gestation.     

Determining Gestational Age in a Low-resource Setting: Validity of Last Menstrual Period

The validity of three methods (last menstrual period [LPM], Ballard and Dubowitz scores) for assessment of gestational age for premature infants in a low-resource setting was assessed, using antenatal ultrasound as the gold standard. It was hypothesized that LMP and other methods would perform similarly in determining postnatal gestational age. Concordance analysis was applied to data on 355 neonates of <33 weeks gestational age enrolled in a topical skin-therapy trial in a tertiary-care children''s hospital in Bangladesh. The concordance coefficient for LMP, Ballard, and Dubowitz was 0.878, 0.914, and 0.886 respectively. LMP and Ballard underestimated gestational age by one day (±11) and 2.9 days (±7.8) respectively while Dubowitz overestimated gestational age by 3.9 days (±7.1) compared to ultrasound finding. LMP in a low-resource setting was a more reliable measure of gestational age than previously thought for estimation of postnatal gestational age of preterm infants. Ballard and Dubowitz scores are slightly more reliable but require more technical skills to perform. Additional prospective trials are warranted to examine LMP against antenatal ultrasound for primary assessment of neonatal gestational age in other low-resource settings.Key words: Gestational age, Last menstrual period, Neonatal health, Obstetrics, Bangladesh     

Assessing number-specific error in the recall of onset of last menstrual period

The goal of this investigation was to determine whether women who did not report preferred numbers for their last menstrual period (LMP) may be a group of women who are particularly careful in keeping track of their menstrual cycles and therefore have more accurate LMP dating--based on a comparison with ultrasound examinations. We also sought to estimate the frequency with which preferred numbers are reported in different sources of data and for different subgroups of women. First, we examined the 1987 California birth certificates in which LMP was collected at the time of birth (n = 504853). We also examined the records of 43880 women participating in the California Alpha-fetoprotein (AFP) Screening Program between 1986 and 1987, for whom gestational ages based on both early ultrasound examination and LMP were collected before 20 weeks of gestation. In the 1987 California birth certificates, seven numbers--1, 5, 10, 15, 20, 25 and 28--were recorded more frequently than expected. An estimated 12.9% of these records had preferred numbers. The most frequently recorded number was 15, occurring 2.5 times more often than expected (P < 0.01). In the data of the AFP Screening Program, the same seven numbers were preferred, and approximately 7.9% of records were affected by number preference. Comparisons with measurements of gestational age based on ultrasound demonstrated that LMP-based gestational ages in which non-preferred numbers are reported for the LMP are slightly more accurate than those in which preferred numbers are reported (P < 0.01). In most cases, number preference appears to introduce small errors into measurements of gestational age, probably as a result of rounding. Thus, the effect of number preference may be primarily of interest to research studies in which small errors in the measurement of gestational age will have a significant impact on findings.     

Assessing the quality of last menstrual period date on California birth records

Birth certificate last menstrual period (LMP) date is widely used to estimate gestational age in the US. While data quality concerns have been raised, no large population-based study has isolated data quality issues by comparing birth record LMP (Birth LMP) with reliable LMP dates from another source. We assessed LMP data quality in 2002 California singleton livebirth records ( n  = 515 381) and in a subset of records with linked prenatally collected LMP from California's statewide Prenatal Expanded Alpha-fetoprotein Screening Program (XAFP) ( n  = 105 936). Missing or incomplete LMP data affected 13% of birth records; 17% of those had complete LMP within XAFP records.
Data quality indicators supported XAFP LMP as more accurate than Birth LMP, with a lower prevalence of digit preference, post-term delivery, out-of-range gestational age estimates and implausible birthweight-for-gestational age. The bimodal birthweight distribution evident at 20–31 weeks' gestation based on Birth LMP was nearly absent with XAFP LMP-based gestational age. Approximately 32% of the second birthweight mode was explained by apparent clerical errors in Birth LMP month. Digit preference errors, particularly day 1, were associated with gestational age overestimation. Preterm delivery rates were higher according to Birth (7.6%) vs. XAFP LMP (7.2%). One-fifth of observed preterm and over half of observed post-term births using Birth LMP were not true cases; 15% of true preterm cases were missed. African American or Hispanic, less educated, and publicly or uninsured women were most likely to be misclassified and have large LMP date discrepancies attributable to clerical or digit preference error. The implementation of a revised birth certificate is an opportunity for targeted training and data entry checks that could substantially improve LMP accuracy on birth records.     

Correction of Systematic Bias in Ultrasound Dating in Studies of Small-for-Gestational-Age Birth: An Example From the Iowa Health in Pregnancy Study

The authors examined whether early ultrasound dating (≤20 weeks) of gestational age (GA) in small-for-gestational-age (SGA) fetuses may underestimate gestational duration and therefore the incidence of SGA birth. Within a population-based case-control study (May 2002-June 2005) of Iowa SGA births and preterm deliveries identified from birth records (n = 2,709), the authors illustrate a novel methodological approach with which to assess and correct for systematic underestimation of GA by early ultrasound in women with suspected SGA fetuses. After restricting the analysis to subjects with first-trimester prenatal care, a nonmissing date of the last menstrual period (LMP), and early ultrasound (n = 1,135), SGA subjects' ultrasound GA was 5.5 days less than their LMP GA, on average. Multivariable linear regression was conducted to determine the extent to which ultrasound GA predicted LMP dating and to correct for systematic misclassification that results after applying standard guidelines to adjudicate differences in these measures. In the unadjusted model, SGA subjects required a correction of +1.5 weeks to the ultrasound estimate. With adjustment for maternal age, smoking, and first-trimester vaginal bleeding, standard guidelines for adjudicating differences in ultrasound and LMP dating underestimated SGA birth by 12.9% and overestimated preterm delivery by 8.7%. This methodological approach can be applied by researchers using different study populations in similar research contexts.     

The research implications of the selection of a gestational age estimation method

There are three primary methods of gestational age estimation: dating based on last menstrual period (LMP), ultrasound-based dating and neonatal estimates. We review the strengths and limitations of each method as well as their implications for research. Dating based on LMP is a simple, low-cost method of estimating gestational age. Limitations associated with the use of menstrual-based dating include reporting problems such as uncertainty regarding the LMP date, possibly due to bleeding not associated with menses, as well as concerns about the incidence of delayed ovulation, which can result in invalid estimates of gestation, even for women with certain LMP dates. Given that most women in the US have at least one ultrasound during pregnancy, it is becoming increasingly common for clinicians to verify menstrual dates using early ultrasound. To calculate gestational age with the use of ultrasound, fetal measurements are compared with a gestational age-specific reference. The primary limitation of this method is the fact that the gestational age estimates of symmetrically large or small fetuses will be biased. Further, given that ultrasound references were developed using pregnancies that were dated according to reliable LMP dates, they are potentially biased in the same direction as dates calculated according to LMP. Neonatal estimates of gestational age have been shown to be the least precise dating method. To highlight the research implications of the choice of a gestational dating method, we used data from the Routine Antenatal Diagnostic Imaging with Ultrasound Study to identify risk factors for post-term delivery. Risk factors for post-term delivery are shown to vary according to the choice of a gestational dating method, suggesting that some findings are an artefact of the choice of a method rather than evidence of causality.     

产前超声检查的布局

目的:探讨超声检查在产前检查中的合理分布。方法:随机抽取4 000例孕妇并将其分成妊娠早期超声检查组和妊娠中期超声检查组各2000例,以排除胎儿畸形和建立预产期,数据通过χ2检验分析进行对照。结果:孕早期超声检查组中,有16%的胎儿畸形是通过孕早期超声检查发现的,且大部分是致死性畸形;通过胎儿顶臀径测定,有36%需修正孕龄。孕中期超声检查组通过胎儿双顶径测定,有8%需修正孕龄。两组相比差异有显著性(P<0.001)。孕早期超声检查组过期妊娠引产率为5%,孕中期超声检查组过期妊娠引产率为12%,两组差异有显著性(P=0.04)。结论:孕早期进行超声检查可早期发现致死性胎儿畸形,有助于减低对预产期判断上的失误,显著降低过期妊娠引产率。     

Variation between last-menstrual-period and clinical estimates of gestational age in vital records

An accurate assessment of gestational age is vital to population-based research and surveillance in maternal and infant health. However, the quality of gestational age measurements derived from birth certificates has been in question. Using the 2002 US public-use natality file, the authors examined the agreement between estimates of gestational age based on the last menstrual period (LMP) and clinical estimates in vital records across durations of gestation and US states and explored reasons for disagreement. Agreement between the LMP and the clinical estimate of gestational age varied substantially across gestations and among states. Preterm births were more likely than term births to have disagreement between the two estimates. Maternal age, maternal education, initiation of prenatal care, order of livebirth, and use of ultrasound had significant independent effects on the disagreement between the two measures, regardless of gestational age, but these factors made little difference in the magnitude of gestational age group differences. Information available on birth certificates was not sufficient to understand this disparity. The lowest agreement between the LMP and the clinical estimate was observed among preterm infants born at 28-36 weeks' gestation, who accounted for more than 90% of total preterm births. This finding deserves particular attention and further investigation.     

Birthweight distributions by gestational age: comparison of LMP-based and ultrasound-based estimates of gestational age using data from the Swedish Birth Registry

Studies based on data from the US have reported that the birthweight distribution at gestational age 28–31 weeks is bimodal with a second peak occurring at approximately 3300 g, suggesting that there is misclassification of term infants. In these studies, gestational ages were estimated from the date of the last menstrual period (LMP), and it has been suggested that ultrasound-based estimates of gestational age would eliminate the problem with bimodal birthweight distributions. Swedish data include both measures, thus offering an opportunity for comparison. All singleton births in Sweden from 1993 to 2002 with information on birthweight were included in the study ( n  = 917 901). Both LMP- and ultrasound-based estimates of gestational age were available for 75.1% of the births. Two possible sources of misclassification were considered: measurement error, assuming that ultrasound-based estimates are better, and data entry errors. An algorithm for assessment of data entry errors was developed; 67.4% of the births were left for the analyses of data 'cleaned' from data entry errors.
Based on the entire study population, the LMP-based birthweight curves for lower-gestational-age preterm births were bimodal, with a second peak around 3500 g. The bimodal distribution was greatly attenuated when using ultrasound-based gestational age categories, but did not disappear. After cleaning the data, the LMP-based birthweight distributions for infants at gestational ages <32 weeks were no longer bimodal, and were very similar to the ultrasound-based curves. In conclusion, data entry errors are more likely to cause the bimodality in the birthweight distribution among preterm infants than measurement errors in the LMP-based gestational age estimate.     

Comparison of gestational age classifications: date of last menstrual period vs. clinical estimate

PURPOSE: The purpose was to compare the two different measures of gestational age currently used on birth certificates (the duration of pregnancy based on the date of last menstrual period [LMP] and the clinical estimate [CE] as related to health status indicators. We contrasted these measures by race/ethnicity. METHODS: NCHS natality files for 2000-2002 were used, selecting cases of single live birth to U.S. resident mothers with both LMP and CE gestational age information. RESULTS: Approximately 75% of the records had valid LMP and CE values and for approximately one-half of these, the LMP and CE values did not exactly agree. Overall and for each race and ethnic group, the LMP measures resulted in higher proportions of very preterm, preterm, postterm and SGA births. CE value provided preterm rates of 7.9% and for LMP, 9.9%. The odds ratio of preterm birth for African-Americans using the CE measure was 1.78 [95% Cl 1.77-1.79]. The odds ratio using LMP was 1.93 [95% Cl 1.92-1.94]. Whites were the referent population. CONCLUSIONS: Different measures of gestational age result in different overall and race-specific rates of very preterm, preterm, postterm, and SGA births. These findings indicate that substituting or combining these measures may have consequences.     

超声检测股骨远端次级骨化中心确定胎龄的临床价值

目的：探讨利用超声检测股骨远端次级骨化中心（SOC）确定胎龄的临床应用价值。方法：对2010年1月-2011年1月在门诊常规产检孕妇700例,选择月经周期不规则或末次月经不详的孕妇350例作为研究组,随机选取月经周期规则的孕妇350例作为对照组,利用超声检测股骨远端SOC的出现时间及最大径线,并分析其与胎龄的关系。结果：孕33周后SOC检出率为95.86%,孕35周检出率100%,大小随胎龄增加而增大。当SOC≥0.6cm时,94.6%的胎儿≥37周。结论：超声检测股骨远端SOC可以方便快捷地确定胎龄,检出SOC在孕33周左右,≥0.6cm时可作为胎龄37周及胎儿成熟的标准。     

Expanding medical abortion: can medical abortion be effectively provided without the routine use of ultrasound?

Medical abortion studies have traditionally relied on ultrasound to confirm gestational age, intrauterine location and abortion completion. However, the routine dependence on ultrasound can limit access to safe services for women living in low resource settings that are often most in need of safe abortion care. This review discusses the literature surrounding the safe provision of medical abortion without the routine use of ultrasonography and concludes that clinicians can use the reported last menstrual period (LMP) and physical examination to reasonably estimate gestational age. Completed pregnancy expulsion can be confirmed primarily through history and physical examination with some studies indicating that urine pregnancy tests may also play a limited role. Central to the discussion of whether medical abortion can be provided in most low resource settings without the routine use of ultrasonography is the fact that the mifepristone–misoprostol regimen is a highly effective procedure for pregnancy termination through 63 days' gestation.     

Evaluation of Gestational Age Estimate Method on the Calculation of Preterm Birth Rates

The objectives of this study is to evaluate the impact of vital record gestational age estimation method on resulting preterm birth (PTB) rate calculations. This retrospective analysis reviewed three methods of gestational age estimation using all Ohio live birth records from 2006 to 2009. PTB rates were calculated using each gestational age representation and agreement between classifications of PTB was evaluated with respect to maternal age and race. For each of 608,530 births, gestational age estimates based on last menstrual period (LMP) were compared to clinically-based obstetric estimates. When gestational age estimates did not perfectly agree, differences in the consequential classification of PTB status were evaluated with respect to a third reconciliatory combined gestational age estimate. Mean birth weight at each week of gestation was calculated and compared for all three estimate methods. Substantial agreement was found in PTB classification among gestational age estimates (kappa: 0.748; 95 % Confidence Interval: 0.745–0.750); agreement was weakest among black mothers and among mothers less than 20 years of age. LMP-based gestational age estimates did not perfectly agree with obstetric estimates in 238,262 records (39.2 %). Disagreement in gestational age led to disagreement in PTB status in 32,033 records (5.3 % of total cases) resulting in a 1.8 percentage point difference in PTB rate calculations (11.0 % using obstetric and 12.8 % using combined estimates). Researchers and policy makers need consistency in selecting which gestational age estimate method to use when calculating or comparing PTB rates.     

成都市孕妇孕期增重与妊娠期糖尿病、孕中期血脂的关系

目的 探讨成都市孕妇孕早、中期增重与妊娠期糖尿病、孕中期血脂情况的关系。方法 选取成都市243例单活胎孕妇作为研究对象,收集其体格状况、个人信息、孕中期血糖、血脂指标（总胆固醇（TC）、甘油三酯（TG）、高密度脂蛋白（HDL）、低密度脂蛋白（LDL））。结果 （1）孕前身体质量指数为消瘦、体重正常和超重肥胖人群的孕早、中期增重量差异均无统计学意义。（2）各期高低增重量人群中,妊娠期糖尿病发病率的差异无统计学意义。（3）对于24～29岁孕妇,孕中期体重增长较高时,其HDL浓度较高。30～35岁孕妇血TC浓度随着孕中期体重增长而增大,另外30～35岁孕妇孕早中期体重增长较高时TG水平也较高。结论 孕妇孕中期及孕早中期增重可能影响孕中期血脂代谢,但孕期增重对血脂代谢的影响存在年龄差异,有待进一步研究。     

太原市新生儿的胎龄相关出生体重分布

目的：构建当地出生体重分布曲线,为临床诊断、流行病学研究和公共卫生决策提供科学依据。方法出生数据来自于太原市出生监测系统,2006-2011年最终符合分析质量要求的单胎、活产儿共计227083例。通过末次月经与分娩日期计算所得胎龄与 B 超诊断胎龄比较,提高胎龄估计的准确性。采用两个高斯分布混合模型鉴别主要分布,应用局部加权回归散点平滑法拟合光滑曲线。结果新生儿出生体重值提供了生物学上合理的平均值、标准差和百分位数值,同时提供了更光滑的百分位数曲线;与以往研究相比,胎龄两端,尤其是小胎龄段,数值趋于稳定;与其它国家的参考值相比,太原的体重值低于欧美研究,与亚洲国家接近。结论基于最新出生人口信息、分性别构建的出生体重分布曲线为新生儿健康状况评价、时空分布和干预措施实施提供参考依据。     

Low prenatal resting energy expenditure and high energy intake predict high gestational weight gain in pregnant women with overweight/obesity

IntroductionRecent evidence suggests that low resting energy expenditure (REE) is associated with gestational weight gain (GWG). However, little research has examined whether REE explains GWG beyond the contributions of energy intake (EI) and physical activity (PA). This study examined the extent to which EI, PA, and REE were associated with and explained second trimester GWG in pregnant women with overweight/obesity.MethodsPregnant women with overweight/obesity (N = 26) participating in the Healthy Mom Zone study, a theoretically-based behavioral intervention that adapted the intervention dosage over time to regulate GWG completed weekly point estimates of EI (back-calculation), PA (wrist-worn activity monitor), and REE (mobile metabolism device) from 14- to 28-weeks gestation. Second trimester GWG was calculated as the weekly point estimate of weight from a Wi-Fi weight scale at gestational week 28 minus the weekly point estimate of weight at gestational week 14.ResultsPartial correlations revealed second trimester EI and PA were not significantly associated with second trimester GWG, but low second trimester REE was significantly associated with high second trimester GWG. Hierarchical regression analyses showed the model of fat-free mass, EI, PA, and REE explained 56% of the variance in second trimester GWG. Low REE was the strongest determinant followed by high EI; fat-free mass and PA were not significant predictors.ConclusionsWhile EI and PA remain important determinants of GWG, future researchers should explore the role of REE to inform individualized EI and PA goals to better regulate GWG.     

Genetic counseling and prenatal care after medications during the first trimester

Analysis of the outcome of 127 pregnancies with first trimester medication (8.4% of the total number of the patients seeking genetic advice in 1997 at the Institute of Medical Genetics in Szeged) was carried out. The gestational age at the time of the medication and genetic counselling, the indications of the treatment, the drugs, the estimated fetal risk, and results of genetic ultrasound examinations and pregnancy outcome were evaluated. The majority of pregnant woman (78%) asked for genetic counselling before the 12. gestational week. The main indications the treatment were: infections, psychiatric-neurologic (depression, anxiety, epilepsy), endocrine (diabetes, hyperthyreoidism), and cardiovascular diseases and gastrointestinal problems. The main groups of the drugs were: antibiotics, antipyretic-, antidepressive-, antidiabetic- and antihypertensive drugs. When the multiple medication was conducted by simultaneous administration of two or more drugs, a complex risk calculation was performed. The fetal risk was higher than 10% in 31 cases (24%). The ultrasound examinations performed by qualified sonographer contributed to a correct evaluation and to reliable follow-up of pregnancies. No suspicious ultrasound finding was reported in the first trimester. However, a severe fatal brain malformation was found in a second trimester pregnancy, which was terminated by the couple's request in the 18th gestational week. A complete follow-up was obtained in 70.9% (90) of the cases. Out of 64 pregnancies intended to continue to term 4 fetal malformations were found. Of them three malformations (patent ductus arteriosus, Robin sequence and a ventricular septal defect) were explored at birth or in the newborn period. The actual 6.3% of fetal malformations was higher compared to the rate expected at birth, but almost equal to the rate of congenital malformation found up to the end of the first year of age in Hungary.     

Trends and differentials in higher-birthweight infants at 28–31 weeks of gestation, by race and Hispanic origin, United States, 1990–2002

Birth certificate gestational age data based on the date of the mother's last menstrual period (LMP) are considered problematic. Of particular concern are birthweight distributions for infants reported on the birth certificate as having been delivered at 28–31 weeks' gestation; these distributions have been shown to be distinctly bimodal. The 'second curve' of the birthweight distribution at 28–31 weeks includes implausible birthweight/gestational age combinations and, thus, has been hypothesised to represent erroneous gestational ages due to misidentification of the date of LMP. It has been suggested that such 'misclassification' has declined in recent years and that this change can affect trends in preterm birth rates (<37 weeks' gestation), particularly rates among non-Hispanic black infants. This present study used primarily simple and multivariable analyses to review trends and differentials in birthweight distributions at 28–31 weeks by race and Hispanic origin of the mother. It aggregated data for the years 1990–92 and 2000–02 from the US vital statistics Natality files.
Over the decade, the percentage of births in the second curve declined for all births and for each racial and Hispanic origin group studied. The largest decline was observed for non-Hispanic blacks; the smallest for Hispanic births. Later initiation of prenatal care, younger maternal age, lower educational attainment, higher birth order and vaginal and singleton delivery were positively associated with a larger second curve, suggesting misclassification of gestational age. Declines in the second curve over the study period were suggested to contribute significantly to the observed decrease in overall preterm birth rates for non-Hispanic black births. Further analysis is needed to estimate the influence of reporting error on preterm birth rates by race and Hispanic origin.     

Comparative accuracy of clinical estimate versus menstrual gestational age in computerized birth certificates

OBJECTIVE: This study compares gestational age data obtained by clinical estimate with data calculated from the date of the last menstrual period (LMP) as recorded on birth certificates. METHODS: The authors analyzed 476,034 computerized birth records from three overlap years, that is, those that contained both menstrual and clinical estimates of gestational age, concentrating on cases within the biologically plausible range of 20-44 weeks. RESULTS: The overall exact concordance between the two measurements was 46%. For +1 week it was 78%, and for +2 weeks it was 87%. Incidence of prematurity was 16% with menstrual gestational age, while it was 12% with clinical estimate. About 47% of the LMP-based preterm births were classified as term by clinical estimate. Eighty-three percent of clinical estimate-based preterms were also preterms by LMP-based gestation. Birthweight frequency distribution curves for LMP-based gestational age are bimodal, indicating probable miscoding of term births. An apparent over-representation of births coded as exactly 40 weeks by clinical estimate suggests rounding off near term for this method. CONCLUSION: Agreement between menstrual and clinical estimates of gestational age occurs most often close to term, with significant disagreement in preterm and postterm births. Use of different methods of determining gestation in different years or geographic populations will result in artifactual differences in important indicators such as prematurity rate.