History and exam

Key diagnostic factors

common

presence of risk factors

Strong risk factors include spontaneous preterm labour, intra-uterine infection, preterm premature rupture of membranes, pre-eclampsia/pregnancy-induced hypertension, abruption/antepartum haemorrhage, abnormal amniotic fluid volume, severe bacterial vaginosis, multiple gestation, previous preterm birth, fetal abnormality, cervical incompetence/uterine abnormality, gestational diabetes, maternal surgery during pregnancy, chronic maternal illness.

maternal last menstrual period (LMP)

Gestational age is estimated as the number of weeks since the LMP ± 2 weeks. LMP gestational age assessment may be incorrect by as much as 2 weeks due to variations in individual menstrual cycle length, recent hormonal contraception, first-trimester bleeding, and inaccurate maternal recollection of dates.

distance from the maternal superior pubis to the uterine fundus

The distance in centimetres from the maternal superior pubis to the uterine fundus is roughly equivalent to the fetal gestational age. However, several factors, such as uterine fibroids or malposition, maternal body habitus, multiple gestation, oligohydramnios or polyhydramnios, fetal growth restriction, or the presence of a small-for-gestational-age infant, may result in an inaccurate assessment of gestational age using this method.

infant physical maturity score

The New Ballard Score uses measurements of physical and neuromuscular maturity to assess gestational age.[31]

Physical factors include examination of the skin, lanugo, plantar surface, breast, eye/ear, and genitals. Each factor is scored between -1 and +4 (or +5 for skin changes). The physical maturity scoring must be combined with the neuromuscular maturity scoring to assess correct gestational age.

infant neuromuscular maturity score

The New Ballard Score uses measurements of neuromuscular and physical maturity to assess gestational age.[31]

Neuromuscular factors include posture, square window, arm recoil, popliteal angle, scarf sign, and heel-to-ear. Each factor is scored between 0 (or -1 for square window, popliteal angle, scarf sign, heel-to-ear) and +4 (or +5 for popliteal angle). The neuromuscular maturity scoring must be combined with the physical maturity scoring to assess correct gestational age.

combined physical/neuromuscular score <10: gestational age <28 weeks

Indicative of extreme prematurity (gestational age <28 weeks). These infants are the most at risk for complications of prematurity.[32]​ On visual inspection, they are very small (birth weight <1000 g).

combined physical/neuromuscular score 11 to 19: gestational age 29 to 31 weeks

Indicates severe prematurity (gestational age 29 to 31 weeks). This sub-group has a birth weight between 1000 g and 1500 g.

combined physical/neuromuscular score 20 to 24: gestational age 32 to 33 weeks

Indicates moderate prematurity (gestational age 32 to 33 weeks). This sub-group has a birth weight between 1500 g and 2000 g.

combined physical/neuromuscular score 25 to 33: gestational age 34 to 36 weeks

Indicates near-term (gestational age 34 to 36 weeks). This sub-group is the most common of the premature groups. The birth weight is usually >2000 g but <2750 g.

Other diagnostic factors

common

infant birth weight (appropriate for gestational age infants)

Infants with gestational age <28 weeks typically have a birth weight of <1000 g.

Infants with gestational age 29 to 31 weeks typically have a birth weight between 1000 g and 1500 g.

Infants with gestational age 32 to 33 weeks typically have a birth weight between 1500 g and 2000 g.

Infants with gestational age 34 to 36 weeks typically have a birth weight of >2000 g but <2750 g.

Risk factors

strong

intra-uterine infection

May account for as much as 40% of preterm births. Atypical micro-organisms (e.g.,Mycoplasma and Ureaplasma) have been implicated.[2]

preterm premature rupture of membranes (PPROM)

Associated with 25% of preterm births. Concurrent intra-uterine infection in some cases may precipitate labour.[2]

pre-eclampsia/pregnancy-induced hypertension

A risk factor for premature birth.[2]

placental abruption/antepartum haemorrhage

Associated with a very high risk of preterm delivery.[2]

abnormal amniotic fluid volume

High (polyhydramnios) or low (oligohydramnios) levels of amniotic fluid are associated with preterm birth and PPROM.[2]

severe bacterial vaginosis

Associated with a onefold to threefold increase in the rate of preterm birth.[2][8]​​

multiple gestation

Between 15% and 20% of all preterm births are multiple gestations, with nearly 60% of twins born preterm; higher for each increasing gestation number. Assisted reproductive technology increases the risk of multiple gestation.[2][8]

previous preterm birth

Results in a twofold to threefold increased risk of preterm delivery for future pregnancies. May be related to concurrent or recurrent intra-uterine infections.[2][8]

fetal abnormality

Common fetal indications for preterm birth include fetal growth restriction, fetal stress, and congenital abnormalities. Fetal abnormalities are associated with 30% of preterm deliveries.[2]

cervical incompetence/uterine abnormality

Preterm delivery is increased due to mechanical reasons related to the abnormal uterine environment or lack of appropriate cervical tone.[2]

gestational diabetes

Poorly controlled gestational diabetes is associated with an increased likelihood of congenital anomalies that increase the risk for preterm delivery.[2]

maternal surgery during pregnancy

Maternal surgery in the second or third trimester can result in preterm labour.[2]

chronic maternal illness

Maternal medical diseases such as diabetes mellitus, asthma, thyroid disease, and chronic hypertension can result in preterm birth due to maternal complications.[2]

Poorly controlled diabetes mellitus is associated with an increased likelihood of congenital anomalies associated with preterm delivery.

maternal pregnancy body mass index <19 or >35

Underweight women (body mass index [BMI] <19) have a higher rate of preterm births than women with a normal BMI.[2][8]​​

The association of obesity (BMI >35) and preterm births is complex. Systematic reviews have reported that obese women have an increased risk of preterm birth.[9][10][11] However, the association differs with maternal age and race. A cohort study conducted in the US reported an inverse association between maternal obesity and preterm birth in white women <20 years and black women <30 years.[12]

weak

short inter-pregnancy time interval

Risk for preterm birth increases to twofold if inter-pregnancy interval is <6 months; potentially related to inadequate time for uterine inflammation to resolve, or inadequate time for repletion of maternal nutritional stores.[2][8]

drug use (tobacco, cocaine, heroin)

Tobacco use during pregnancy increases the preterm risk up to twofold. Cocaine and heroin use are also associated with preterm delivery.[2][8]

stress/depression

Presence of these factors increases the risk of preterm birth onefold to twofold. May be related to increased systemic inflammation or concurrent drug use/abuse.[2]

extremes of maternal age

Studies show that women at extremes of maternal age (>35 or <24 years old) are at higher risk of delivering a neonate who is born preterm.[13][14][15][16][17]​​​​​​​ However, one retrospective case control study concluded that, with good perinatal care, women aged 45 years and older may have similar perinatal and neonatal risks compared to younger mothers.[18]​ 

non-white race

Black women have a higher risk of preterm delivery compared with white women.[4][8]​ Black women are 3 to 4 times more likely to have a very early preterm birth compared with other racial and ethnic groups. The mechanisms behind this difference are unclear.[2]

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