Conditions · Obstetrics and gynecology
Post-term pregnancy !
A pregnancy that lasts 42 weeks or more. The placenta can start to fail, so the baby is monitored closely and labour is often induced.
Updated 2026-11-01
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Why it happens
What it is
A pregnancy that reaches or passes 42 weeks 0 days (294 days) from the first day of the last menstrual period. Pregnancies from 41 weeks 0 days to 41 weeks 6 days are called late term. Accurate dating, ideally with first-trimester ultrasound, is essential before the label is applied.
Who gets it
A few percent of pregnancies reach 42 weeks; the proportion is lower where early ultrasound dating and routine induction at 41 weeks are standard. Colombian antenatal guidelines recommend offering induction from 41 weeks.
Causes
- Wrong dating (the most common reason for an apparently post-term pregnancy)
- Most true cases have no known cause
- Rare fetal causes: anencephaly, placental sulfatase deficiency, fetal adrenal hypoplasia
Risk factors
- Previous post-term pregnancy
- First pregnancy
- Maternal obesity
- Male fetus
- Uncertain last menstrual period without early ultrasound
Pathophysiology
Labour onset depends on fetal and placental signals (fetal adrenal cortisol, placental oestrogen and prostaglandins). When these signals are delayed, pregnancy continues while the ageing placenta becomes less efficient. Two paths follow: if the placenta keeps working, the fetus keeps growing (macrosomia, shoulder dystocia); if it fails, oxygen and nutrient delivery falls, fetal urine output drops (oligohydramnios), the cord is easily compressed, and hypoxia can trigger meconium passage.
Classification
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post-term: 42 weeks 0 days or more
- Postmaturity syndrome in the newborn (dry peeling skin, long nails, little fat, meconium staining)
Symptoms and signs
- No onset of labour after the due date
- Reduced fetal movements in some cases
- Fundal height smaller than expected if oligohydramnios develops
Diagnosis
- Clinical
- Based on gestational age of 42 weeks or more, calculated from a reliable last menstrual period and confirmed or corrected by early ultrasound. Bishop score assesses how ready the cervix is for induction.
- Tests
- Fetal surveillance from 41 weeks: nonstress test and biophysical profile.
- Imaging
- Ultrasound for amniotic fluid volume (deepest pocket or index), estimated fetal weight and presentation.
Differential diagnosis
- Incorrect dating
- Fetal growth restriction (small uterus)
- Fetal macrosomia
Goal of treatment
Avoid stillbirth and neonatal harm by monitoring the fetus from 41 weeks and offering delivery before or at 42 weeks.
Treatment
- Procedure
- Membrane sweeping from about 39 to 41 weeks to increase the chance of spontaneous labour.
- Medical
- Induction of labour offered from 41 weeks, with cervical ripening (prostaglandins or a balloon catheter) when the cervix is unfavourable and oxytocin infusion afterwards.
- Follow-up
- Antenatal fetal testing (nonstress test and fluid volume) for women who choose to wait, and continuous fetal heart monitoring during induced labour.
- Surgical
- Caesarean section for usual obstetric indications such as fetal distress or failed induction.
- Follow-up
- Teach daily fetal movement awareness, monitor contractions and fetal heart rate during induction, and prepare for meconium and shoulder dystocia at birth.
Complications
- Stillbirth and neonatal death (risk rises after 41 weeks)
- Meconium aspiration syndrome
- Oligohydramnios and cord compression
- Macrosomia, shoulder dystocia and birth trauma
- Postmaturity syndrome and neonatal hypoglycaemia
- Maternal perineal tears, prolonged labour and postpartum haemorrhage
Prognosis
With accurate dating, surveillance and timely induction, most babies are born healthy. Perinatal risk rises steadily beyond 41 weeks, which is why routine induction is offered at that point.
Prevention
- First-trimester ultrasound to date the pregnancy accurately
- Offering induction from 41 weeks
- Membrane sweeping at term visits
Test yourself
Think of the answer, then tap the question to see it.
What gestational age defines a post-term pregnancy?
42 weeks 0 days or more.
What is the most common reason for an apparently post-term pregnancy?
Incorrect dating.
Which amniotic fluid change suggests placental failure after term?
Oligohydramnios.
What are two fetal risks of post-term pregnancy?
Meconium aspiration and stillbirth (also macrosomia with shoulder dystocia).
Which score is used to assess the cervix before induction?
The Bishop score.
Related conditions
- Amniotic fluid disorders (polyhydramnios, oligohydramnios)
- Stillbirth (intrauterine fetal death)
- Meconium aspiration syndrome
- Shoulder dystocia
- Labour dystocia (arrest of labour, cephalopelvic disproportion)
- Intrapartum fetal compromise (non-reassuring fetal status)
- Perinatal asphyxia and hypoxic-ischaemic encephalopathy
- Postpartum haemorrhage
- Fetal growth restriction
- Neonatal hypoglycaemia (including infant of a diabetic mother)
- Obesity
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Inducing labor (MedlinePlus (U.S. National Library of Medicine), 2024); Amniotic fluid (MedlinePlus (U.S. National Library of Medicine), 2024).
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