(632) Ultrasound Alone versus Last Menstrual Period for Gestational Age Assignment: Implications for Fetal Growth Restriction

Thursday, January 30, 2025

4:00 PM – 6:00 PM

Submitting Author and Presenting Author(s)

Shae Boguslawski, MD (she/her/hers)

OBGYN resident
Henry Ford Health System, Department of Obstetrics and Gynecology
Detroit, Michigan, United States

Coauthor(s)

ET

Elise Turke, BS

Wayne State University School of Medicine
Wayne State University School of Medicine/Detroit, Michigan, United States
RP

Rebekah Pitpitan, BS

Wayne State University School of Medicine
Wayne State University School of Medicine/Detroit, Michigan, United States
KO

Kyra Orton, BS

Michigan State University College of Human Medicine
Michigan State University College of Human Medicine/East Lansing, Michigan, United States
MS

Majid Shaman, MD

MFM attending
Henry Ford Health
Detroit, Michigan, United States
SK

Sun Kwon Kim, MD, PhD

MFM Attending
Henry Ford Health
Detroit, Michigan, United States
GG

Gregory Goyert, MD

Division Head, Maternal Fetal Medicine
Henry Ford Health
Detroit, Michigan, United States

Objective: Accurate pregnancy dating is crucial for evaluating fetal growth and diagnosing fetal growth restriction (FGR). In the first trimester, both the last menstrual period (LMP) and crown-rump length (CRL) are typically used to determine gestational age. Our primary objective was to compare the rates of FGR diagnosed using LMP and CRL with those diagnosed using CRL alone. The secondary objective was to assess the differences in pregnancy outcomes between these two groups.

Study Design: This cohort study utilized an ultrasound database including all singleton pregnancies with a first-trimester ultrasound from 2015 to 2023. We identified two groups of FGR: those dated by LMP and CRL and those dated by CRL alone. FGR was diagnosed based on an estimated fetal weight or fetal abdominal circumference below the 10th percentile. Exclusion criteria included cases without a first-trimester ultrasound (gestational age < 14 weeks 0 days), multiple pregnancies, in vitro fertilization pregnancies, and cases lacking a delivery record.

Results: We identified 1067 pregnancies affected by FGR in our cohort (n=14116): 733 out of 9251 pregnancy episodes were dated by LMP and CRL, and 334 out of 4865 were dated by CRL alone. The rate of FGR was higher in those dated by LMP and CRL compared to those dated by CRL alone (7.9% vs 6.8%, p</span><0.001). A higher rate of FGR dated by LMP and CRL resolved by the end of pregnancy compared to FGR dated by CRL alone (28.6% vs 21.0%, p=0.008). There were no significant differences between the groups in mean gestational age at delivery, mode of delivery, preterm births, severe FGR, abnormal umbilical artery Dopplers, and composite adverse neonatal complications.

Conclusion: Although FGR diagnosed by CRL alone did not show statistically significant differences in pregnancy outcomes, the lower rate of FGR in this group suggests a potential over-diagnosis when using both LMP and CRL. Future research should focus on the impact of modifying gestational age determination methods to potentially reduce the false positive rate of FGR and unnecessary antepartum interventions.