Post-term Pregnancy (Prolonged Pregnancy)

Post-term pregnancy is defined as: "Delivery on or after 41 weeks plus three days of gestation, 10 days beyond the estimated date of delivery."

Where possible, first trimester ultrasound rather than last menstrual period (LMP) dating should be relied on to assess pregnancy duration. This should be determined using crown-rump measurement or head circumference if crown-rump length is above 84 mm.[1]

This PatientPlus article is written for healthcare professionals so the language may be more technical than the condition leaflets. You may find the abbreviations list helpful.

  • Post-term pregnancies are associated with both fetal and maternal complications.[2] 
  • However, although the fetal, maternal and neonatal risks increase beyond 41 weeks, there is no conclusive evidence that prolongation of pregnancy per se is the major risk factor.
  • Other risk factors for adverse outcomes with post-term pregnancy are often restricted fetal growth and fetal malformations.
  • Beyond around 41 weeks, placental function may decline and become insufficient, reducing the supply of oxygen and nutrients to the fetus.
  • There is also increased risk of meconium aspiration syndrome and neonatal hypoglycaemia.
  • Compared with delivery at 40 weeks of gestation, delivery at 42 weeks or later has been shown to be associated with an increased risk of cerebral palsy.[3]
  • The risk of Caesarean delivery and maternal complications also increase with gestational age.
  • There is increased risk of fetal macrosomia, ie birth weight >4 kg and birth injury.
  • Some fetal anomalies - eg, anencephaly, are associated with prolonged pregnancy.
  • The use of ultrasound in early pregnancy for precise dating significantly reduces the number of post-term pregnancies compared to dating based on the LMP.[4]
  • Around 20% of pregnant women will need induction of labour - the majority for post-term pregnancy
  • 74% of women will have delivered by 40 weeks of gestation, 74% by 41 weeks and 82% by 42 weeks
  • Post-term pregnancy is now one of the most common indications for induction of labour.

Risk factors

  • Previous post-term pregnancy increases the risk of recurrence in subsequent pregnancies. 
  • High maternal BMI is associated with longer gestation and increased rate of induction of labour.[5]
  • Primigravidity.
  • Elevated pre-pregnancy weight and maternal weight gain both increase the risk of a post-term delivery.[6]
  • Advanced maternal age is also a strong risk factor for prolonged pregnancy.[7]

Symptoms

  • When post-term, 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.

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.

Management of post-term pregnancy in the absence of other complications is controversial:

  • The Royal College of Obstetricians and Gynaecologists (RCOG)/National Institute for Health and Clinical Excellence (NICE) guidelines recommend that women should be offered induction after 41 weeks between 41+0 and 42+0 weeks to avoid the risks of post-term pregnancy, primarily increased intrauterine fetal death.[8]
  • Women should have a  cardiotocography (CTG) undertaken at around term plus ten days to ensure the fetus' well-being. A vaginal examination should also be undertaken to determine the modified Bishop's score and to act as a baseline against which to compare subsequent examinations.  The Bishop score consists of five parameters (including dilation and effacement of cervix) which are assessed by a vaginal examination. A higher score increases the likelihood of having a successful induction and vaginal delivery.
  • A recent study showed that induction of labour in obese women with post-term pregnancy is a safe management option and a reasonable way of avoiding Caesarean section.[5]
  • One recent systematic review showed that a policy of labour induction for women with post-term pregnancy compared with expectant management is associated with fewer perinatal deaths and fewer Caesarean sections.[9] 
  • Many units now offer routine induction at around 41 weeks. 
  • In women without any other risk factors, it may sometimes be appropriate to let women make an informed decision about which management they wish to undertake.
  • Close intrapartum fetal surveillance in all these women should be offered, irrespective of whether labor was induced or not.
  • The RCOG recommends that women should be given information to ensure they fully understand the choices they are making regarding accepting or refusing induction of labour. This should include the reasons for induction, the method to be used and any alternatives, as well as any potential risks and consequences of accepting or declining induction of labour.
  • Written information for patients to take home with them to read is important.
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Further reading & references

  1. Antenatal care: routine care for the healthy pregnant woman, NICE Clinical Guideline (March 2008)
  2. Doherty L, Norwitz ER; Prolonged pregnancy: when should we intervene? Curr Opin Obstet Gynecol. 2008 Dec;20(6):519-27.
  3. Moster D, Wilcox AJ, Vollset SE, et al; Cerebral palsy among term and postterm births. JAMA. 2010 Sep 1;304(9):976-82.
  4. Mandruzzato G, Alfirevic Z, Chervenak F, et al; Guidelines for the management of postterm pregnancy. J Perinat Med. 2010 Mar;38(2):111-9.
  5. Arrowsmith S, Wray S, Quenby S; Maternal obesity and labour complications following induction of labour in prolonged pregnancy. BJOG. 2011 Apr;118(5):578-88. doi: 10.1111/j.1471-0528.2010.02889.x. Epub 2011 Jan 26.
  6. Halloran DR, Cheng YW, Wall TC, et al; Effect of maternal weight on postterm delivery. J Perinatol. 2012 Feb;32(2):85-90. doi: 10.1038/jp.2011.63. Epub 2011 Jun 16.
  7. Roos N, Sahlin L, Ekman-Ordeberg G, et al; Maternal risk factors for postterm pregnancy and cesarean delivery following labor induction. Acta Obstet Gynecol Scand. 2010 Aug;89(8):1003-10.
  8. Induction of labour, NICE Clinical Guideline (July 2008)
  9. Gulmezoglu AM, Crowther CA, Middleton P, et al; Induction of labour for improving birth outcomes for women at or beyond term. Cochrane Database Syst Rev. 2012 Jun 13;6:CD004945.
Original Author: Dr Hayley Willacy Current Version: Peer Reviewer: Prof Cathy Jackson
Last Checked: 13/12/2012 Document ID: 2640  Version: 22 © EMIS

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