Pregnancy Complications Increase Risk of Future Preterm Birth

Liv Kvalvik, MD, PhD

If a woman experienced pregnancy complications—despite having their first baby born at full term‑there was an increased risk of preterm delivery for their next pregnancy, study findings showed.

The findings suggested that complications could help identify women at increased risk of preterm delivery.

Liv Kvalvik, MD, PhD, and colleagues from Norway and North Carolina included 302,192 women giving birth to a second singleton child between 1999-2015. Data were obtained from the population-based Medical Birth registry of Norway. Collected data included demographic; medical and reproductive history; lifestyle; pre-pregnancy and prenatal information, complications during pregnancy and delivery; and fetal and infant outcomes.

Kvalik, from the Department of Global Public Health and Primary Care at the University of Bergen in Norway, and the investigative team excluded those with missing information on gestational age and birthweight, along with women with babies who weight >5 standard deviations above the mean for gestational week of birth (.1%).

The team’s main outcome was preterm birth in the second delivery—liveborn or stillborn—at 20-36 gestational weeks. Gestational age was completed weeks and was based on the date of the woman’s last menstrual period.

The investigators identified 5 complications or poor outcomes of term pregnancy: pre-eclampsia; placental abruption; stillbirth; neonatal death; and small for gestational age.

The risk of preterm birth in the first pregnancy was 5.9%. For those with term birth in their first pregnancy, 4.2% had preeclampsia, .2% had placental abruption; .2% had stillbirth; .1% had neonatal death; and 2.4% had a small for gestational age baby.

Risk of recurrence of preterm birth in the second pregnancy was 18.1% (relative risk, 5.5%; 95% CI, 5.3-5.7) compared with a term first birth. Each of the 5 complications was linked with a substantially increased risk of preterm birth in the second pregnancy.

Absolute risks for preterm birth in the second pregnancy were 3.1% with none of the complications, 6.1% after pre­eclampsia, 7.3% after placental abruption, 13.1% after stillbirth, 10% after neonatal death, and 6.7% after small for gestational age.

The adjusted relative risk of preterm birth after preeclampsia was 2.0 (95% CI, 1.8-2.1); 2.3 after placental abruption (95% CI, 1.7-3.1); 4.2 after stillbirth (95% CI, 3.4-5.2); 3.2 after neonatal death (2.2-4.8); and 2.2 after small for gestational age (95% CI, 2-2.4).

Having any 1 of the complications was linked to a doubled risk of the next birth being preterm (RR, 2; 95% CI, 1.9-2.1) (absolute risk, 6.2%). The relative risk with >2 complications was 3.5 (95% CI, 2.9-4.2) (absolute risk, 10.9%).

Although women who have a term pregnancy generally would be considered at reduced risk for subsequent preterm birth, the findings emphasized that complications could lead to a substantially increased future risk.

The findings may inform antenatal clinical care by helping to identify women at increased risk of preterm delivery.

Additional research of the causal factors underlying the risks could lead to insight of biological mechanisms linked to a broad range of pregnancy complications.

The study, “Term complications and subsequent risk of preterm birth: registry based study,” was published online in The BMJ.