Surprising New Research on the Risks for Preterm Birth

The independent risk factors for spontaneous preterm birth (SPTB) with intact membranes (SPTB-IM) and SPTB after prelabor rupture of membranes (SPTB-PPROM) for healthy nulliparous pregnant women are markedly different, according to results of a new study.¹

Between 11% and 12% of the 4 million neonates born each year are delivered before 37 weeks, and 3.6% are delivered before 34 weeks.² To date, the only screening test for SPTB with any potential for effective intervention is measurement of cervical length.³

Of 3184 women in the Screening for Pregnancy Endpoints (SCOPE) study for whom follow-up data was gathered, 156 (4.9%) had SPTB (96 had SPTB-IM and 60 had SPTB-PPROM). The primary outcome for this study was birth before 37 weeks’ gestation.

The independent risk factor most strongly correlated with SPTB-IM was a family history of low birth weight babies (OR, 5.64). Other risk factors for SPTB-IM were shorter cervical length, abnormal uterine Doppler flow at 20 weeks’ gestation, regular use of marijuana pre-pregnancy, lack of overall feeling of well being, white ethnicity, and having a mother with diabetes or a history of preeclampsia.

Surprisingly, mild hypertension that does not require treatment was the independent risk factor most strongly associated with SPTB-PPROM (OR, 9.65). A family history of recurrent gestational diabetes mellitus was also strongly correlated (OR, 8.01). Other risk factors for SPTB-PPROM were shorter cervical length, short stature, the participant’s not being the first born in the family, longer time to conceive, not waking up at night, hormonal fertility treatment (excluding clomiphene), and low BMI. Another unexpected finding was that maternal family history of any miscarriage was associated with a risk reduction. Some of these independent risk factors for SPTB-PPROM are difficult to explain and may represent false discoveries for some of the findings, according to the study authors.

Decreased cervical length and BMI were the only variables present in both models. According to the study authors, this was the first study to link greater maternal height with protection from SPTB-PPROM but not SPTB-IM and the first to show that mild hypertension is a risk factor for SPTB-PPROM.

These independent risk factors can only modestly predict SPTB, and risk markers cannot be translated into any useful clinical tool for daily practice. However, these findings provide additional insight into these conditions, said the authors. Taking a complete family history can provide potentially important indicators for SPTB risk.

Pertinent Points:
- The dissimilarity of risk factors for SPTB-IM compared with SPTB-PPROM indicates different pathophysiological pathways underlie these distinct phenotypes.
- Despite knowing these independent risk factors, their ability to predict preterm birth in healthy nulliparous women is modest.

References:

1. Dekker GA, Lee SY, North RA, et al. Risk factors for preterm birth in an international prospective cohort of nulliparous women. PLoS ONE. 2012;7:e39154. doi:10.1371/journal.pone.0039154.
2. Mathews TJ, MacDorman MF. Infant mortality statistics from the 2007 period linked birth/infant death data set. Natl Vital Stat Rep. 2011;59:1-30.
3. Fonseca E, Celik E, Parra M, et al. Progesterone and the risk of preterm birth among women with a short cervix. N Engl J Med. 2007;357:462-469.