Etiology
Factors associated with preterm birth may be classified as modifiable or nonmodifiable.
Modifiable factors include:[2]
A short interpregnancy time interval: a twofold risk increase in preterm birth if there is <6 months between pregnancies.
A maternal pregnancy BMI <19 or >35.
Psychological or social stress and maternal depression increase the risk of preterm birth onefold to twofold.
Tobacco use during pregnancy increases the risk of preterm birth onefold to twofold. Cocaine and heroin use are also associated with preterm delivery.
Intrauterine infection may account for as much as 40% of preterm births. Atypical microorganisms (e.g., Mycoplasma and Ureaplasma) have been implicated.
Severe bacterial vaginosis is associated with a onefold to threefold increase in the rate of preterm birth.
Poorly controlled maternal illness such as diabetes mellitus (or gestational diabetes) is associated with an increased likelihood of congenital anomalies that may lead to preterm delivery. Poorly controlled maternal hypertension may also lead to preterm delivery.
Nonmodifiable factors include:[2]
Race: black women have a higher risk of preterm delivery compared with white women. Black women are 3 to 4 times more likely to have a very early preterm birth than other racial and ethnic groups.
Previous preterm birth: results in a twofold to threefold increased risk of preterm delivery for future pregnancies.
Chronic medical diseases such as diabetes, asthma, thyroid disease, and hypertension. Maternal surgery in the second or third trimester can result in preterm labor.
Maternal causes: preeclampsia/pregnancy-induced hypertension, antepartum hemorrhage, oligohydramnios or polyhydramnios, cervical or uterine abnormality.
Fetal causes: multiple gestation, fetal growth restriction, some fetal anomalies.
Preterm premature rupture of membranes (PPROM) is associated with preterm birth. Concurrent intrauterine infection frequently precipitates labor.
Development of preterm spontaneous labor is a major cause of preterm births, due to the limited ability to stop labor once it is progressive.
Between 30% and 35% of preterm births occur as a result of fetal or maternal conditions, 40% to 45% are due to spontaneous labor with intact membranes, and 25% to 30% are due to PPROM.[2]
Pathophysiology
Pathways associated with preterm birth include excessive myometrial and fetal membrane distension, decidual hemorrhage, precocious fetal endocrine activation, and intrauterine infection or inflammation.[7] These mechanisms, acting in concert or independently, result in preterm labor and in premature birth at various gestational ages. Intrauterine infection associated with preterm premature rupture of membranes (PPROM), and preterm labor is typically associated with extreme prematurity.[8]
Classification
Based on gestational age[2]
Extreme prematurity
less than 28 weeks' gestation
Severe prematurity
28 to 31 weeks' gestation
Moderate prematurity
32 to 33 weeks' gestation
Late-preterm infants
34 to 36 weeks' gestation
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