History and exam
Key diagnostic factors
common
presence of risk factors
Risks of premature labour are greater with a previous history of preterm birth, previous cervical surgery, previous emergency caesarean section or induced abortion, infection, multifetal pregnancies, and positive fetal fibronectin.
uterine contractions
There are no specific thresholds at which the frequency of contractions becomes significant; even regular contractions are not associated with labour in most cases. Uterine tightening is a normal physiological finding, and perception is highly variable. However, the more symptomatic and more frequent the contractions, the more likely they will lead to delivery. They cannot be relied upon to positively predict labour, but contraction frequencies of >1 in 10 minutes are less likely to be physiological Braxton-Hicks contractions.
preterm prelabour rupture of membranes (PPROM)
Fetal membranes will rupture spontaneously in labour in most women, including those in premature labour. However, in more than one third of preterm women, rupture will occur prior to the onset of symptomatic contractions.[29] This is associated with a higher risk of maternal and fetal infection (both as a cause and consequence of PPROM). Pooling of the liquor may be seen on speculum examination.
advanced cervical dilation
Cervical dilation makes premature labour highly likely. In conjunction with regular uterine contractions, labour is diagnosed. A closed cervix is consistent with threatened premature labour.
cervical length <2 cm
On speculum examination of the cervix, a short cervical length is associated with increased risk of premature labour. This can be confirmed by transvaginal ultrasound.
Other diagnostic factors
uncommon
increased maternal or fetal heart rate
This may occur in response to infection.
non-specific lower abdominal or back pain
Atypical presentations may include non-specific abdominal or back pain.
fever
Systemic fever of any cause, including malaria and listeriosis, can result in onset of premature labour.
vaginal bleeding
This may indicate antepartum haemorrhage due to a placental abruption. It is usually associated with pain and uterine activity and contractions.
Risk factors
strong
previous premature labour
Risk of premature labour is greater for women who have had a previous preterm delivery.[17] One previous preterm delivery increases the risk 4-fold, rising to 6.5-fold with two previous preterm deliveries. The gestational age at delivery also affects the risk: the earlier the delivery, the higher the risk of recurrence.
However, the absolute level of risk rarely exceeds 50%, even in women with the worst previous histories, suggesting that even high-risk women can have a successful pregnancy. Women should be appropriately counselled regarding this level of risk. Truly recurrent causes of premature labour are rare.
Previous iatrogenic preterm birth also increases the risk of subsequent spontaneous preterm birth, probably owing to placental pathology, which may recur in subsequent pregnancies with different clinical manifestations.[41]
previous cervical trauma
There is an established relationship between previous cervical surgery and future risk of preterm birth.[42] Laser conisation, radical diathermy, large loop excisions, and emergency caesarean section may all be associated with higher risks of adverse events including perinatal mortality.[23][25] Laser ablation and cryotherapy are not associated with increased risk. Women who have undergone cold knife conisation have a significantly higher risk of severe preterm delivery compared with women who have not undergone any surgery.[42] Some retrospective evidence suggests that the treatment itself may not be as important as the underlying disease.[43] The risk appears to correlate with the depth of excision.[40][44]
previous induced abortion
Women with a history of previous induced abortion also have an increased risk of premature labour, particularly for deliveries before 28 weeks' gestation.[27]
maternal infections
Urinary tract infections, including asymptomatic bacteriuria, have a strong association with premature labour, and treatment results in a significant reduction in the incidence of pyelonephritis and low birth weight, although a fall in preterm birth rates was not shown.[13]
Abnormal vaginal flora, particularly bacterial vaginosis found early in pregnancy, is associated with higher risk of spontaneous premature labour.[14] However, antimicrobial treatment does not have a significant impact on the likelihood of preterm delivery.[15]
In one study, ear-nose-throat infection in early pregnancy was associated with an increased risk of spontaneous preterm delivery.[45] COVID-19 infection is associated with an increased risk of preterm birth.[46][47]
Systemic infections, such as malaria or listeriosis, may also cause preterm labour.
multifetal pregnancies
About 60% of twins are born preterm, and 19.5% are born before 34 weeks’ gestation.[34] Nearly all triplet and higher-order multiple pregnancies are preterm due to uterine stretch.
Iatrogenic preterm delivery is considerably higher in this group, due to higher rates of growth restriction and other complications.
short cervical length
A short cervical length (<2 cm) places a woman at higher risk of preterm delivery.
positive fetal fibronectin test
preterm prelabour rupture of membranes (PPROM)
In more than one third of preterm women, rupture will occur prior to the onset of symptomatic contractions.[29] This is associated with a higher risk of maternal and fetal infection (both as a cause and a consequence of PPROM).
weak
fetal abnormalities
Common fetal indications for premature labour include fetal growth restriction, fetal stress, and congenital abnormalities. Fetal abnormalities are associated with 8% of preterm deliveries (including iatrogenic preterm delivery).[35]
smoking
There is a strong dose-response relationship between tobacco smoking and premature labour, although it is difficult to establish causation. There is also some evidence that stopping smoking between pregnancies reduces the risk of a preterm birth.[21] Underlying mechanisms are unclear but, as smoking has also been associated with intrauterine growth restriction, it is likely to contribute to premature labour and is discouraged during pregnancy. In 2016, 7.2% of pregnant women in the US smoked during pregnancy.[51] In England, around 7.5% of mothers are smokers at the time of delivery.[52]
One retrospective cohort study in Canada reported that cannabis use during pregnancy was associated with an increased risk of premature birth (RR 1.41, 95% confidence interval [CI] 1.36-1.47 compared with the matched cohort).[53]
body mass index (BMI) <19 kg/m²
Low maternal weight is associated with increased risk of early delivery. Higher rates of spontaneous premature labour are associated with low BMI. At a BMI of <19, absolute risk of spontaneous delivery is 16.6%, compared with 8.1% in those with a normal BMI (19-25 kg/m²).[54] These studies also suggest that rates of spontaneous preterm birth are lower for women with obesity, but iatrogenic causes of preterm birth are increased with obesity, possibly related to the associated oxidative stress, particularly as a result of pre-eclampsia.[24]
social factors and ethnicity
Many factors linked to social disadvantage are related to premature labour, including education, marital status, and low income.[18] In addition, recreational drugs, alcohol, caffeine, and psychological stress have all been linked to early birth.[19] These may also be related to maternal factors such as null parity, low maternal age, and ethnic origin. However, most epidemiological data sets do not distinguish between spontaneous and iatrogenic causes of early birth.[55] The mechanism for preterm birth in these populations is not clear, and even within different ethnic groups there are confounding factors. A higher proportion of preterm deliveries occurs in women of black ethnicity and Asian women of Indian origin.[6]
fertility treatment
One large population-based cohort study found an increased risk of premature birth in singleton neonates conceived by assisted reproductive technology (adjusted odds ratio 1.49) and other fertility treatments (adjusted odds ratio 1.35).[56] The mechanism is unclear.
polyhydramnios
Can result in spontaneous premature birth.
domestic violence
A prospective study of 16,000 women presenting to a labour unit showed that the incidence of low birth-weight infants was significantly increased in women who reported domestic verbal abuse compared with those who did not. The rates of neonatal deaths were higher in those who reported domestic physical abuse. Women who declined to be interviewed also had increased rates of low birth-weight infants and of preterm births at <32 weeks' gestation compared with women in the no-abuse group.[22]
poor dental hygiene
late-stage caesarean section
A sixfold increased risk of preterm birth has been linked to late-stage caesarean section in the US. Among women who had a caesarean section during the second stage of labour, 13.5% had a subsequent preterm birth, compared with 2.3% in women who had a first-stage caesarean section.[57] Cohort studies from the UK and Australia have also reported increased risk of subsequent spontaneous preterm birth in women with late-stage caesarean section, compared with those with early-stage caesarean section or vaginal birth.[25][26][58][59]
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