Quick answer: At 39 weeks your baby is fully developed and ready to be born, measuring around 50.7cm and weighing about 3.3 kilograms. If labour hasn't started by 40 weeks, your midwife will offer a membrane sweep, and induction is offered at 41 or 42 weeks because the risk of complications increases slightly after 42 weeks. The decision about induction is always yours.
What parents need to know
You've made it to 39 weeks — your baby could arrive any day now! The vast majority of babies are born between 38 and 42 weeks, with the average being around 40 weeks for first-time mothers and slightly earlier for subsequent babies. This week, let's talk about what happens if your due date comes and goes, and what options are available.
| Baby's length | About 50.7cm crown to heel |
|---|---|
| Baby's weight | Around 3.3 kilograms, like a large watermelon |
| Typical birth window | Most babies born between 38 and 42 weeks |
| Membrane sweep offered | At 40 weeks if no signs of labour |
| Induction offered | At 41 or 42 weeks without labour beginning |
How big is your baby at 39 weeks?
Your baby now measures approximately 50.7cm from crown to heel and weighs around 3.3 kilograms — roughly the weight of a large watermelon. They are fully developed and ready to be born, with all the systems they need for life outside the womb in place.
What's happening with your baby?
Your baby's brain continues to develop — a process that will continue for years after birth. Their skin is soft and smooth, and most of the vernix caseosa (the white waxy coating) has been absorbed. Your baby is in a state of quiet readiness, conserving energy for the demanding work of birth and the newborn period. Their lungs are fully mature, their digestive system is ready, and their liver is prepared to process waste products after birth.
Understanding your options if labour doesn't start
Most women go into labour naturally before or by 42 weeks, but some pregnancies go beyond the due date. If you reach 40 weeks without signs of labour, your midwife will offer a membrane sweep. This is an internal examination during which the midwife uses a gloved finger to sweep around the cervix and detach the membranes from the cervix wall. This releases hormones (prostaglandins) that can trigger labour. It can be mildly uncomfortable but is safe and can reduce the need for medical induction.
If you reach 41 or 42 weeks without labour beginning, you'll be offered induction of labour. This is because the risk of complications, including stillbirth, increases slightly after 42 weeks. Induction methods include prostaglandin gel or pessary applied to the cervix, a hormone drip (Syntocinon/oxytocin) to start contractions, or artificial rupture of the membranes (ARM) to break the waters. Your midwife and obstetrician will discuss the risks and benefits with you, and the decision about induction is always yours to make.
Choosing to wait for spontaneous labour
Some women choose to decline or delay induction. If this is your preference, your care team should support you with increased monitoring, including fetal heart rate monitoring and scans to check amniotic fluid levels. The RCOG recommends that all options are discussed openly and that women are supported to make an informed decision that reflects their individual circumstances and values.
Your week 39 checklist
This week:
- Keep monitoring your baby's movements and call immediately if they reduce.
- Ask about a membrane sweep at your appointment.
- Rest, rest, rest.
- Know that most babies arrive by 42 weeks at the latest, with induction available if needed.
Frequently asked questions
What is a membrane sweep and does it hurt?
It's an internal examination where the midwife sweeps a gloved finger around the cervix to detach the membranes, releasing prostaglandins that can trigger labour. It can be mildly uncomfortable but is safe, and may reduce the need for medical induction.
Can I decline induction at 41 or 42 weeks?
Yes — the decision about induction is always yours. Some women choose to decline or delay it. If so, your care team should support you with increased monitoring, including fetal heart rate monitoring and scans to check amniotic fluid levels.
What induction methods are used in the UK?
Methods include prostaglandin gel or a pessary applied to the cervix, a hormone drip (Syntocinon/oxytocin) to start contractions, or artificial rupture of the membranes (ARM) to break the waters.
Sources & further reading
- NHS — You and Your Baby at 39 Weeks Pregnant
- RCOG — Induction of Labour at Term
- Tommy's — Week 39 of Pregnancy
- NCT — Overdue Babies: Your Options
General information, not individual medical advice. Speak to your GP, midwife or health visitor about your baby’s needs.
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