Table 2

Relative risk of preterm birth (PTB)* in second pregnancy by term complications in first pregnancy with and without exclusion of recurrent complications, Norway 1999-2015

First pregnancy (at term)PTB in second pregnancyPTB in second pregnancy (recurrences excluded†)
No with PTB/No at risk (%)Relative risk (95% CI)No with PTB/No at risk (%)Relative risk (95% CI)
No complications‡8202/265 043 (3.1)Reference
Pre-eclampsia§688/11 225 (6.1)2.0 (1.8 to 2.1)7069/244 496 (2.9)Reference
404/9696 (4.2)1.4 (1.3 to 1.6)
Placental abruption§41/562 (7.3)2.3 (1.7 to 3.1)7463/247 527 (3.0)Reference
35/547 (6.4)2.1 (1.5 to 2.9)
Stillbirth72/551 (13.1)4.2 (3.4 to 5.2)7712/264 247 (2.9)Reference
69/548 (12.6)4.3 (3.5 to 5.4)
Neonatal death22/219 (10.0)3.2 (2.2 to 4.8)7547/263 945 (2.9)Reference
22/218 (10.1)3.5 (2.4 to 5.2)
SGA 2.5¶463/6939 (6.7)2.2 (2.0 to 2.4)7994/259 629 (3.1)Reference
426/5900 (7.2)2.3 (2.1 to 2.6)
  • * Birth between 20 and 36 gestational weeks.

  • Excludes both exposed and unexposed women with same complication in 2nd pregnancy. Therefore, number at risk in reference group changes slightly for each exposure.

  • For main analyses reference is term first pregnancies with none of the five complications. For analysis excluding recurrence of the same complication, reference further excludes women with the complication of interest in the second pregnancy.

  • § Reference category “No complications” for term pre-eclampsia and placental abruption is (number with PTB in 2nd pregnancy/number at risk) 7750/248 214 (3.1%) because the analytical period is restricted to 1999-2014 for these analyses.

  • Small for gestational age (SGA) 2.5th centile by each term gestational week (weeks 44-46 grouped). To avoid the possibility of data entry issues, term births with infants with birth weight less than 1000 g were excluded (n=5) from analyses on term SGA, and preterm infants in second pregnancy with birth weight ≤100 g were excluded (n=8).