How many months is 40 weeks pregnant?

40 weeks is roughly 9 months. Months are an awkward unit in pregnancy because they do not divide evenly into weeks — a calendar month averages about 4.3 weeks, which is why 40 weeks works out a little over nine months rather than ten. Appointments and scans are always booked in weeks, so weeks are the figure worth carrying.

How big is a baby at 40 weeks?

The NHS publishes around 51.2 cm long, from head to heel for week 40. Rather than one fruit, that length is offered here against eight rulers, each object carrying its own measurement: the sweetest thing you will ever make (51.2 cm head to heel), a newborn human, the least camouflaged mammal on the continent (51.2 cm head to heel), a 51.2 cm original, no reissue (51.2 cm head to heel), a debutant, yet to be selected for anything (51.2 cm head to heel), and four more. Published lengths are averages across many babies, not a measurement of any particular one, and a scan estimate that differs is a normal thing rather than a finding.

What trimester is 40 weeks?

Week 40 is in the third trimester, on the commonly published split of weeks 1–12, 13–26 and 27 onward. The boundaries are conventions for talking about pregnancy rather than biological switches — nothing changes overnight at week 13 or week 27.

40 weeks pregnant

What happens at 40 weeks pregnant?

At 40 weeks a baby is around 51.2 cm long, from head to heel, about the size of the sweetest thing you will ever make. That is roughly 9 months, in the third trimester, the week a due date is set for.

Published length

51.2 cm long, from head to heel

In months

Roughly 9 months

Trimester

third · due now

What is developing at 40 weeks

The estimated due date. Only about 1 in 20 babies arrive on it — most come somewhere between 37 and 42 weeks. The baby carries on growing and laying down fat right up to labour, and the skull bones stay unfused so the head can mould on the way out.

The size comparison above is the sweetest thing you will ever make, which measures 51.2 cm head to heel. Every comparison on this site is a real object with a real measurement, checked against the published length. The same week is drawn in seven other sets below, from Australian animals to fast food, if this one means nothing to you.

40 weeks, in eight rulers

Every other pregnancy tool measures a baby in fruit. This one offers eight sets to choose from, and each object below carries its real measurement, checked against the 51.2 cm the NHS publishes for week 40. Pick whichever ruler you actually have a feel for. Why not fruit.

Sweet treats
the sweetest thing you will ever make
51.2 cm head to heel
Australian animals
a newborn human, the least camouflaged mammal on the continent
51.2 cm head to heel
The 90s
a 51.2 cm original, no reissue
51.2 cm head to heel
Sport
a debutant, yet to be selected for anything
51.2 cm head to heel
Things you already own
a newborn baby
51.2 cm head to heel
Gaming
a new player, entering the game
51.2 cm head to heel
Fast food
the only takeaway worth a nine-month wait
51.2 cm head to heel
Toys
the only toy that plays back
51.2 cm head to heel

Your dates, from 40 weeks

Set to someone who is exactly 40 weeks today, so the due date below is the one that implies. Change the date to your own and everything follows from it — nothing is saved unless you do.

How far along am I?

Your due date

Do not know your due date yet? The due date calculator estimates one from your last period. A dating ultrasound is more accurate than any calendar method — if your pregnancy care team has given you a date, use theirs.

Questions about 40 weeks

How many months is 40 weeks pregnant?: 40 weeks is roughly 9 months. Months are an awkward unit in pregnancy because they do not divide evenly into weeks — a calendar month averages about 4.3 weeks, which is why 40 weeks works out a little over nine months rather than ten. Appointments and scans are always booked in weeks, so weeks are the figure worth carrying.
How big is a baby at 40 weeks?: The NHS publishes around 51.2 cm long, from head to heel for week 40. Rather than one fruit, that length is offered here against eight rulers, each object carrying its own measurement: the sweetest thing you will ever make (51.2 cm head to heel), a newborn human, the least camouflaged mammal on the continent (51.2 cm head to heel), a 51.2 cm original, no reissue (51.2 cm head to heel), a debutant, yet to be selected for anything (51.2 cm head to heel), and four more. Published lengths are averages across many babies, not a measurement of any particular one, and a scan estimate that differs is a normal thing rather than a finding.
What trimester is 40 weeks?: Week 40 is in the third trimester, on the commonly published split of weeks 1–12, 13–26 and 27 onward. The boundaries are conventions for talking about pregnancy rather than biological switches — nothing changes overnight at week 13 or week 27.

Where these numbers come from

NHS — Week-by-week guide to pregnancy — The published length of a baby at each week of pregnancy — measured head to bottom until week 20 and head to heel from week 20 — and what is developing at that stage.
Pregnancy, Birth and Baby — Pregnancy week by week — The Australian Government health service’s week-by-week account of how a pregnancy is counted and what develops at each stage.

General information only, not medical advice. BubSync is not a medical device: these tools do not diagnose, treat or monitor any condition, and using one does not create a clinical relationship. Every baby is different, and these are published ranges rather than targets to hit — they may not apply to your baby, particularly if they were born preterm, are small or large for their age, or have a medical condition. Never delay or disregard advice from a health professional because of something a calculator here showed you. If you are worried about feeding, sleep, weight or wet nappies, contact your child health nurse, midwife or GP — and if your baby seems unwell, floppy, or is difficult to rouse, seek urgent medical care now (000 in Australia).

Written by the BubSync editorial team. Not yet reviewed by an independent clinician. The editorial policy says who writes these pages and how their figures are sourced. Read how we write and check these pages.

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