The association between customised small for gestational age infants and pre-eclampsia or gestational hypertension varies with gestation at delivery

Objectives (1) To describe the association between small for gestational age (SGA) infants and pre-eclampsia (PE) and gestational hypertension (GH) and (2) to determine how this association changes with gestational age at delivery using customised centiles to classify infants as SGA. Design A retrospective observational study. Setting National Women's Hospital, a Tertiary Referral Centre in Auckland, New Zealand. Population A total of 17 855 nulliparous women delivering between 1992 and 1999. Methods A comparison of the number of women with a customised SGA infant, PE and GH according to gestational age at delivery. Main outcome measures The incidence of SGA infants (defined as birthweight < 10th customised centile), PE and GH at < 34, 34-36(+6) and >= 37 weeks. Results A total of 1847 (10.3%) infants were SGA, 520 (2.9%) women had PE and 1361 (7.6%) had GH. SGA, PE and GH all occurred more commonly with increasing gestation at delivery with 85%, 62% and 90% of cases delivered at term. In women delivering SGA infants, coexisting PE was more likely to occur among those delivered preterm than at term (38.6% at < 34 weeks [relative risk, RR 10.2 95%CI 7.3-14.4], 22.4% at 34-36(+6) weeks [RR 6.0 95%CI 4.1-8.6] and 3.8% at >= 37 weeks [OR 1.0]). Women with preterm PE were more likely to have a SGA infant than women with term PE (57.1% at < 34 weeks [RR 3.1 95%CI 2.3-4.2], 31.7% at 34-36(+6) weeks [RR 1.7 95%CI 1.2-2.5]) and 18.3% at >= 37 weeks [OR 1.0]). There was a similar association between GH and SGA infants as gestation advanced (57.6% at < 34 weeks [RR 4.8 95%CI 3.4-6.6], 30.5% at 34-36(+6) weeks [RR 2.5 95%CI 1.8-3.5] and 12.1% >= 37 weeks [OR 1.0]). Conclusions SGA infants and PE are more likely to coexist in preterm births compared with term births. This is likely to reflect the degree of placental involvement in each disease process.