Small fetal size before 20 weeks' gestation: associations with maternal tobacco use, early preterm birth, and low birthweight

OBJECTIVE: The objective of the study was to determine whether small fetal size before 20 weeks' gestation is associated with preterm birth (PTB), low birthweight ( LBW), and poor fetal growth. STUDY DESIGN: A total of 4405 singleton pregnancies at 10-19 weeks' gestation (GA) based on a known last menstrual period (LMP) were evaluated. Ultrasound-estimated GA (US-GA) was calculated based on crown-rump length at 10-13 weeks and by femur, head, and abdominal measurements from 14 to 19 weeks. The outcomes were compared between small (1-10 days smaller than LMP-GA) and large (0-10 days larger than LMP-GA) fetuses. RESULTS: At 10-19 weeks, small fetuses measured 2.7 days younger and were more likely to have mothers who smoked ( P = .004). Small fetuses had no more PTB ( 11.4 vs 12.1%, P = .47) but did have more early PTB before 34 (5.4 vs 4.3%, P = .07) and before 32 weeks (4.1 vs 2.7%, P = .009). Small fetuses had lower birthweights (BWT), more frequent BWT below 2500 g (13.0 vs 8.6%), below 1500 g (4.0 vs 2.4%), and below 1000 g (2.9 vs 1.4%) as well as BWT below 2500 g at term (4.9 vs 2.3%) and BWT less than the 10th percentile (8.8 vs 3.7%), P <= .003 for each. Small fetuses at 10-19 weeks also had less frequent macrosomia and were less frequently large for gestational age at birth ( P < .0001 for each). These findings largely persisted in multivariable analyses. CONCLUSION: Small fetal size at 10-19 weeks is associated with tobacco use in pregnancy, early PTB, LBW, and poor fetal growth.