A comparison of LMP‐based and ultrasound‐based estimates of gestational age using linked California livebirth and prenatal screening records

Abstract

Although early ultrasound (<20 weeks' gestation) systematically underestimates the gestational age of smaller fetuses by approximately 1-2 days, this bias is relatively small compared with the large error introduced by last menstrual period (LMP) estimates of gestation, as evidenced by the number of implausible birthweight-for-gestational age. To characterise this misclassification, we compared gestational age estimates based on LMP from California birth certificates with those based on early ultrasound from a California linked Statewide Expanded Alpha-fetoprotein Screening Program (XAFP). The final sample comprised 165 908 women. Birthweight distributions were plotted by gestational age; sensitivity and positive predictive value for preterm rates according to LMP were calculated using ultrasound as the 'gold standard'.For gestational ages 20-27 and 28-31 weeks, the LMP-based birthweight distributions were bimodal, whereas the ultrasound-based distributions were unimodal, but had long right tails. At 32-36 weeks, the LMP distribution was wider, flatter, and shifted to the right, compared with the ultrasound distribution. LMP vs. ultrasound estimates were, respectively, 8.7% vs. 7.9% preterm (<37 weeks), 81.2% vs. 91.0% term (37-41 weeks), and 10.1% vs. 1.1% post-term (Ն42 weeks). The sensitivity of the LMPbased preterm birth estimate was 64.3%, and the positive predictive value was 58.7%. Overall, 17.2% of the records had estimates with an absolute difference of >14 days. The groups most likely to have inconsistent gestational age estimates included African American and Hispanic women, younger and less-educated women, and those who entered prenatal care after the second month of pregnancy. In conclusion, we found substantial misclassification of LMP-based gestational age.The 2003 revised US Standard Certificate of Live Birth includes a new gestational age item, the obstetric estimate. It will be important to assess whether this estimate addresses the problems presented by LMP-based gestational age.

United States Birth Weight Reference Corrected For Implausible Gestational Age Estimates

“…Previous work suggests that the second mode primarily reflects errors in LMP dating. 5,7,24 However, the accuracy of obstetric/clinical estimates should not be implicitly assumed. With ultrasound-based data, …”

“…The need for such a reference has been specifically expressed in the literature, as concerns regarding the validity of any analysis based solely on LMPs have become increasingly appreciated. 5,7,11,24 Comparisons with other references, along with extensive data presented here and in the online supplement, will allow researchers and clinicians to weigh the appropriateness of our reference against their specific needs.…”

Maternal serum progesterone, estradiol and estriol levels in successful dinoprostone-induced labor

“…In the present study, gestational age was estimated by early ultrasound examination (,20 weeks), since last menstrual period-based (LMP-based) gestational age involves high rates of gestational age misclassification (24). Literature data show that inaccuracy of LMP-based gestational age can be caused by biologically associated errors in menstrual cycles and by human error in recall or data entry (25).…”

Post-Natal Gestational Age Assessment Using Sonographic Measurement of Femoral Length

“…Early trimester ultrasound has been established clinically as the gold standard 14 but only 52% will have three antenatal visits and most of the women seek medical care 1 st time at the time of delivery in which case GA estimation by any method is not possible 15 .…”