History and exam
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Prévention de l’accouchement prématuré chez les femmes à risque – évaluation de quelques mesures courantesPublished by: KCELast published: 2014Preventie bij verhoogd risico op vroeggeboorte - evaluatie van een aantal courante interventiesPublished by: KCELast published: 2014Key diagnostic factors
common
history of preterm labor, cervical trauma, or induced abortion
Risk of preterm labor (including iatrogenic preterm delivery) is greater for women who have had a previous preterm delivery.[17] The earlier the gestational age at preterm delivery, the higher the risk of recurrence.
Laser conization, radical diathermy, and large loop excisions may all be associated with higher risks of adverse events including perinatal mortality.[23] Laser ablation and cryotherapy are not associated with increased risk. Women who have undergone cold knife conization have a significantly higher risk of severe preterm delivery compared with women who have not undergone any surgery.[41]
Women with a history of previous induced abortion also have an increased risk of preterm labor, particularly for deliveries before 28 weeks' gestation.[27]
multifetal pregnancy and presence of maternal infection
Twin pregnancies will deliver 3 weeks early, on average, and nearly all triplet and higher-order multiple pregnancies are preterm.
UTIs, including asymptomatic bacteriuria, have a strong association with preterm labor, and abnormal vaginal flora, particularly bacterial vaginosis found early in pregnancy, is associated with higher risk of spontaneous preterm labor.[14] Systemic infections, such as malaria or listeria, may also cause preterm labor.
uterine contractions
There are no specific thresholds at which the frequency of contractions becomes significant; even regular contractions are not associated with labor in most cases. Uterine tightening is a normal physiologic 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 labor, but contraction frequencies of >1 in 10 minutes are less likely to be physiologic Braxton-Hicks contractions.
preterm prelabor rupture of membranes (PPROM)
Fetal membranes will rupture spontaneously in labor in most women, including those in preterm labor. 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 amniotic fluid may be seen on speculum exam.
advanced cervical dilation
Cervical dilation makes preterm labor highly likely. In conjunction with regular uterine contractions, labor is diagnosed. A closed cervix is consistent with threatened preterm labor.
cervical length <2 cm
On speculum examination of the cervix, a short cervical length is associated with increased risk of preterm labor. This can be confirmed by transvaginal ultrasound.
Other diagnostic factors
uncommon
increased maternal or fetal heart rate
This may occur in response to infection.
nonspecific lower abdominal or back pain
Atypical presentations may include nonspecific abdominal or back pain.
fever
Systemic fever of any cause, including malaria and listeriosis, can result in onset of preterm labor.
vaginal bleeding
This may indicate antepartum hemorrhage due to a placental abruption. It is usually associated with pain and uterine activity and contractions.
Risk factors
strong
previous preterm labor
Risk of preterm labor 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 2 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 counseled regarding this level of risk. Truly recurrent causes of preterm labor 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.[40]
previous cervical trauma
There is an established relationship between previous cervical surgery and future risk of preterm birth.[41] Laser conization, radical diathermy, large loop excisions, and emergency cesarean 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 conization have a significantly higher risk of severe preterm delivery compared with women who have not undergone any surgery.[41] Some retrospective evidence suggests that the treatment itself may not be as important as the underlying disease.[42] The risk appears to correlate with the depth of excision.[39][43]
previous induced abortion
Women with a history of previous induced abortion also have an increased risk of preterm labor, particularly for deliveries before 28 weeks' gestation.[27]
maternal infections
Urinary tract infections, including asymptomatic bacteriuria, have a strong association with preterm labor, 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 preterm labor.[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.[44] COVID-19 infection is associated with an increased risk of preterm birth.[45][46]
Systemic infections, such as malaria or listeriosis, may also cause preterm labor.
multifetal pregnancies
About 60% of twins are born preterm, and 19.5% are born before 34 weeks’ gestation.[33] 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 prelabor 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 preterm labor include fetal growth restriction, fetal stress, and congenital abnormalities. Fetal abnormalities are associated with 8% of preterm deliveries (including iatrogenic preterm delivery).[34]
smoking
There is a strong dose-response relationship between tobacco smoking and preterm labor, 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 preterm labor and is discouraged during pregnancy. In 2016, 7.2% of pregnant women in the US smoked during pregnancy.[50]
One retrospective cohort study in Canada reported that cannabis use during pregnancy was associated with an increased risk of preterm birth (RR 1.41, 95% CI 1.36 to 1.47 compared with the matched cohort).[51]
body mass index (BMI) <19 kg/m²
Low maternal weight is associated with increased risk of early delivery. Higher rates of spontaneous preterm labor 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).[52] 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 preeclampsia.[24]
social factors and ethnicity
Many factors linked to social disadvantage are related to preterm labor, 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 epidemiologic data sets do not distinguish between spontaneous and iatrogenic causes of early birth.[53] 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 preterm birth in singleton neonates conceived by assisted reproductive technology (adjusted odds ratio 1.49) and other fertility treatments (adjusted odds ratio 1.35).[54] The mechanism is unclear.
polyhydramnios
Can result in spontaneous preterm birth.
domestic violence
A prospective study of 16,000 women presenting to a labor 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 cesarean section
A sixfold increased risk of preterm birth has been linked to late-stage cesarean section in the US. Among women who had a cesarean section during the second stage of labor, 13.5% had a subsequent preterm birth, compared with 2.3% in women who had a first-stage cesarean section.[55] Cohort studies from the UK and Australia have also reported increased risk of subsequent spontaneous preterm birth in women with late-stage cesarean section, compared with those with early-stage cesarean section or vaginal birth.[25][26][56][57]
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