Preterm birth aetiology 2004-2008. Maternal factors associated with three phenotypes: spontaneous preterm labour, preterm pre-labour rupture of membranes and medically indicated preterm birth.

Objectives: To (1) investigate the current distribution of PTB phenotypes; (2) identify factors associated with spontaneous preterm labour (SPTL), PPROM, and indicated PTB; (3) investigate the relationship of gestational age (ga) with each PTB phenotype.

Methods: Retrospective review of all live, singleton births 23(+0) to 36(+6) weeks ga at an obstetric referral centre 2004-2008.

Results: A total of 4,522 PTBs were included (SPTL 31.7%, PPROM 27.4%, indicated 40.8%). PTB phenotype distribution differed between ga groups (<27 weeks: SPTL 45%, PPROM 32%, indicated 23%; 27-33 weeks: SPTL 30%, PPROM 32%, indicated 39%; 34-36 weeks: SPTL 32%, PPROM 24%, indicated 44%, p < 0.001). Between 34-36 weeks', demographic factors were significantly different between PTB phenotypes (age ≥35: SPTL 13.8%, PPROM 15.4%, indicated 21.6%; Caucasian ethnicity: SPTL 61.6%, PPROM 69.0%, indicated 70.2%; Assisted Reproductive Technology (ART): SPTL 2.8%, PPROM 1.9%, indicated 9.3%; all p < 0.001). Between 27-33 weeks' PTB phenotype was associated with smoking (SPTL 24.9%, PPROM 29.3%, indicated 20.2%; p = 0.002) and ART (SPTL 2.3%, PPROM 1.6%, indicated 5.0%; p = 0.002). Demographic factors were not associated with PTB phenotype at 23-26 weeks.

Conclusions: The increase in PTB rates may be explained by medical indications at late preterm gestations, primarily in older, Caucasian women requiring fertility assistance. Interventions to reduce the rate of PTB need to be targeted to this high-risk population.