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Table 1 Risk factors associated with an increased risk of preterm birth and the effectiveness of intervention arrayed according to the strength of evidence

From: Born Too Soon: Care before and between pregnancy to prevent preterm births: from evidence to action

   How great is the risk?
Pregnancy in adolescence + Increased prevalence of anemia, pregnancy-induced hypertension, low birthweight, prematurity, intra-uterine growth retardation and neonatal mortality
Birth spacing +  
Short intervals   PTb: OR 1.45, LBW: OR 1.65
Long intervals   PTb: OR 1.21, LBW: OR 1.37
Pre-pregnancy weight status +  
Underweight   PTb: OR 1.32, LBW: OR 1.64
Overweight & obesity   PTb: OR 1.07
   Maternal overweight is a risk factor for many pregnancy complications including hypertensive disorders, gestational diabetes, postpartum hemorrhage, stillbirth, congenital disorders
Both underweight and overweight women have a higher chance for requiring obstetric intervention at delivery
Micronutrient deficiencies +/-  
Folic acid   Folic acid deficiency is definitively linked to neural tube defects (NTDs) in newborns
Iron   Anemia increases the risk for maternal mortality, low birthweight, preterm birth and child mortality
Chronic diseases +  
Diabetes mellitus   Babies born to women with diabetes before conception have a much higher risk of stillbirths, perinatal mortality, congenital disorders, as well as spontaneous pregnancy loss, preterm labor, hypertensive disorders, and delivery by cesarean birth.
Anemia   A study shows that anemia before conception increases the risk of low birthweight (OR 6.5)
Poor mental health (especially depression) and Intimate partner violence ++ Increased risk for preterm birth, low birthweight and depression during pregnancy and the postpartum period
IPV-PTb OR 1.37, LBW OR: 1.17
Also increased risk for spontaneous pregnancy loss, stillbirth, gynecological problems including sexually-transmitted infections, depression
Infectious diseases ++  
STIs - syphilis   Infectious diseases increase the risk for spontaneous pregnancy loss, stillbirths and congenital infection
Tobacco use ++ A single study shows risk PTb OR: 2.2
Smoking increases the risk for spontaneous pregnancy loss, placental disorders, congenital malformations, sudden infant death syndrome, stillbirths and low birth weight
  1. For magnitude of risk:
  2. ++ means strong evidence of risk and implicated in biological pathways leading to preterm birth and low birthweight
  3. + means moderate evidence of risk on preterm birth and low birthweight
  4. +/- means weak evidence of risk on preterm birth and low birthweight
  5. Acroynms used: PTb = preterm birth; OR = odds ratio; IPV = intimate partner violence
  6. Source: Barros et al., 2010; Bhutta et al., 2011a; Goldenberg et al., 2008; Iams et al., 2008