Determinants of Small for Gestational Age Birth at Term

Background:   Being born small for gestational age (SGA) is an indicator of intrauterine growth restriction (IUGR) and later health risks. This study investigated determinants of severe and moderate SGA (respectively, birthweight <3rd percentile and 3rd to <10th percentile for gestational age and sex).

Methods:   A total of 2195 term pregnancies from a prospective cohort were studied. Prenatal data arose from maternal interview at 10-22 weeks of gestation and perinatal data were collected from hospital charts. Severe and moderate SGA were classified by Canadian population standards. Risk factors for SGA were identified from fitting multivariable logistic regression models.

Results:   Multivariable associations with severe SGA were: maternal age ≥ 35 [odds ratio (OR) 3.2 [95% confidence interval (CI) 1.4, 6.9]], maternal smoking during pregnancy (OR 5.3 [95% CI 2.4, 11.7]), preeclampsia (OR 4.6 [95% CI 1.6, 13.2]) and threatened preterm labour (OR 3.9 [95% CI 1.3, 11.4]). Primiparity was associated with both severe and moderate SGA with OR 2.4 [95% CI 1.1, 5.1] and OR 1.9 [95% CI 1.3, 2.9] respectively. Underweight pre-pregnancy body mass index was associated with moderate SGA (OR 2.4 [95% CI 1.2, 5.0]). Inclusion of placental weight, in the final model attenuated the associations.

Conclusions:   This study demonstrated different determinants for severe and moderate SGA. We speculate that the majority of severe SGA infants are IUGR while moderate SGA infants may be a mixture of IUGR and constitutionally small newborns. This study has also contributed evidence linking preterm labour and SGA as two, potentially related, outcomes of overlapping causal mechanisms reflective of ischaemic placental disease.