Perinatal risks associated with prolonged pregnancy

• Beyond around 41 weeks placental function may decline and become insufficient, reducing the supply of oxygen and nutrients to the fetus. Placental insufficiency increases the risk of intrapartum fetal hypoxia.
• There is also increased risk of meconium aspiration syndrome and neonatal hypoglycaemia.
• The risk of stillbirth or neonatal death (in healthy women with normal pregnancies) is greater at 42 weeks than 37 weeks. The risk has been shown to be up to 8 times greater at 43 weeks.^2,3
• The risk of Caesarean delivery and maternal complications also increase with gestational age.
• There is increased risk of fetal macrosomia, i.e. birth weight > 4 kg and birth injury.
• Some fetal anomalies, e.g. anencephaly, are associated with prolonged pregnancy.
• Increased risk of epilepsy in the neonate, particularly if delivered by instrumental delivery or Caesarean section.⁴

Epidemiology

Approximately 7.5% of pregnancies continue to 42 weeks or beyond.⁵Postmaturity is a syndrome seen in some infants born at or after 42 weeks. However, the term post-mature is often used to describe any infant born after 42 weeks.

Risk factors

Previous prolonged pregnancy increases risk of recurrence in subsequent pregnancies two- to three-fold.⁶
Few pre-natal risk factors are known. However recent work suggests an association with:

• BMI > 35.⁷
• Primigravidity.
• Fish consumption in first 2 trimesters.⁸

Presentation

Symptoms

• When post-mature the neonate has lower than normal amounts of subcutaneous fat and reduced mass of soft tissue.
• The skin may be loose, flaky and dry.
• Fingernails and toenails may be longer than usual and stained yellow from meconium.

Signs

• Before delivery there may be reduced fetal movement.
• A reduced volume of amniotic fluid may cause a reduction in the size of the uterus.
• Meconium-stained amniotic fluid may be seen when the membranes have ruptured.

Investigations

Women with no other indications for induction, who do not wish labour to be induced, can be offered monitoring to assess placental function and fetal health. There is a lack of evidence with which to assess the benefits of monitoring and the effectiveness of the various techniques.⁹

Management

Management of prolonged pregnancy in the absence of other complications is controversial:

• The Royal College of Obstetricians and Gynaecologists/NICE guidelines recommend that women should be offered induction after 41 weeks.¹⁰
• Women who decline induction should be offered increased antenatal monitoring from 42 weeks, consisting of twice-weekly cardiotocography (CTG) and ultrasound estimation of single deepest amniotic pool. A pool depth of < 8 cm indicates increased intrapartum risk to the fetus.¹¹
• If expectant management is used, some sources recommend labour should be induced at the beginning of the 43rd week.⁹

However, in a recent randomised trial there were no differences between induced (at 289 days) and monitored groups (every 3 days) in neonatal morbidity, mode of delivery, and general outcome.¹²