Aetiology
Factors associated with preterm birth may be classified as modifiable or non-modifiable.
Modifiable factors include:[2]
A short inter-pregnancy 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.
Intra-uterine infection may account for as much as 40% of preterm births. Atypical micro-organisms (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.
Non-modifiable 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 labour.
Maternal causes: pre-eclampsia/pregnancy-induced hypertension, antepartum haemorrhage, 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 intra-uterine infection frequently precipitates labour.
Development of preterm spontaneous labour is a major cause of preterm births, due to the limited ability to stop labour 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 labour 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 haemorrhage, precocious fetal endocrine activation, and intra-uterine infection or inflammation.[6] These mechanisms, acting in concert or independently, result in preterm labour and in premature birth at various gestational ages. Intra-uterine infection associated with preterm premature rupture of membranes (PPROM), and preterm labour is typically associated with extreme prematurity.[7]
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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