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 > or =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 > or =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 > or =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% > or =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.