Term complications and subsequent risk of preterm birth: registry based study

Abstract

Objective: To explore conditions and outcomes of a first delivery at term that might predict later preterm birth.

Design: Population based, prospective register based study.

Setting: Medical Birth Registry of Norway, 1999-2015.

Participants: 302 192 women giving birth (live or stillbirth) to a second singleton child between 1999 and 2015.

Main outcome measures: Main outcome was the relative risk of preterm delivery (<37 gestational weeks) in the birth after a term first birth with pregnancy complications: pre-eclampsia, placental abruption, stillbirth, neonatal death, and small for gestational age.

Results: Women with any of the five complications at term showed a substantially increased risk of preterm delivery in the next pregnancy. The absolute risks for preterm delivery in a second pregnancy were 3.1% with none of the five term complications (8202/265 043), 6.1% after term pre-eclampsia (688/11 225), 7.3% after term placental abruption (41/562), 13.1% after term stillbirth (72/551), 10.0% after term neonatal death (22/219), and 6.7% after term small for gestational age (463/6939). The unadjusted relative risk for preterm birth after term pre-eclampsia was 2.0 (95% confidence interval 1.8 to 2.1), after term placental abruption was 2.3 (1.7 to 3.1), after term stillbirth was 4.2 (3.4 to 5.2), after term neonatal death was 3.2 (2.2 to 4.8), and after term small for gestational age was 2.2 (2.0 to 2.4). On average, the risk of preterm birth was increased 2.0-fold (1.9-fold to 2.1-fold) with one term complication in the first pregnancy, and 3.5-fold (2.9-fold to 4.2-fold) with two or more complications. The associations persisted after excluding recurrence of the specific complication in the second pregnancy. These links between term complications and preterm delivery were also seen in the reverse direction: preterm birth in the first pregnancy predicted complications in second pregnancies delivered at term.

Conclusions: Pre-eclampsia, placental abruption, stillbirth, neonatal death, or small for gestational age experienced in a first term pregnancy are associated with a substantially increased risk of subsequent preterm delivery. Term complications seem to share important underlying causes with preterm delivery that persist from pregnancy to pregnancy, perhaps related to a mother's predisposition to disorders of placental function.

Fig 1

Flowchart of study population

Fig 2

Unadjusted (filled diamonds) and adjusted (open diamonds) relative risks for preterm birth in second pregnancy by complications in first pregnancy at term, Norway, 1999-2015. Reference is term birth in first pregnancy without any of the five complications. Analyses are adjusted for maternal age, year of birth for first child, and maternal education. Supplementary eTable 1 presents adjusted relative risks (95% confidence intervals) with further adjustments. SGA=small for gestational age

Fig 3

Relative risks (95% confidence intervals) for birth in specific gestational weeks in second pregnancy for women with any term complications in first pregnancy, Norway 1999-2015. Exposure is any term complication (one or more of five complications). Reference is term birth in first pregnancy without any of the five complications. Because pre-eclampsia and placental abruption are included in the “Any complication” category, analysis is restricted to 1999-2014. Denominators are all fetuses in utero at the beginning of the specific week interval. With 100% of births occurring in the final interval (≥43 weeks), no estimates were calculated for these last gestational weeks. Filled circles represent unadjusted relative risks for birth in specific gestational weeks in the second pregnancy