How many months is 18 weeks pregnant?

18 weeks is roughly 4 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 18 weeks?

The NHS publishes around 14.2 cm long, from head to bottom for week 18. Rather than one fruit, that length is offered here against eight rulers, each object carrying its own measurement: a chocolate bar, the sharing-size lie (14 cm end to end), a superb fairy-wren (14 cm beak to tail), a Game Boy (14.8 cm top to bottom), a tungsten dart (15 cm point to flight), 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 18 weeks?

Week 18 is in the second 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.

18 weeks pregnant

What happens at 18 weeks pregnant?

At 18 weeks a baby is around 14.2 cm long, from head to bottom, about the size of a chocolate bar, the sharing-size lie. That is roughly 4 months, in the second trimester, with about 22 weeks to go until 40.

Published length

14.2 cm long, from head to bottom

In months

Roughly 4 months

Trimester

second · 22 weeks to go

What is developing at 18 weeks

The baby can hear — a heartbeat, the rumble of digestion, and voices from outside, though muffled. Myelin, the fatty sheath that insulates nerves and speeds their signals up, is beginning to coat them. Yawning and hiccupping become regular.

The size comparison above is a chocolate bar, the sharing-size lie, which measures 14 cm end to end. 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.

18 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 14.2 cm the NHS publishes for week 18. Pick whichever ruler you actually have a feel for. Why not fruit.

Sweet treats
a chocolate bar, the sharing-size lie
14 cm end to end
Australian animals
a superb fairy-wren
14 cm beak to tail
The 90s
a Game Boy
14.8 cm top to bottom
Sport
a tungsten dart
15 cm point to flight
Things you already own
the phone in your hand
14.7 cm top to bottom
Gaming
a Nintendo DS, closed
14.8 cm side to side
Fast food
a hot dog
15 cm end to end
Toys
a Transformers figure, robot mode
14 cm head to feet

Your dates, from 18 weeks

Set to someone who is exactly 18 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 18 weeks

How many months is 18 weeks pregnant?: 18 weeks is roughly 4 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 18 weeks?: The NHS publishes around 14.2 cm long, from head to bottom for week 18. Rather than one fruit, that length is offered here against eight rulers, each object carrying its own measurement: a chocolate bar, the sharing-size lie (14 cm end to end), a superb fairy-wren (14 cm beak to tail), a Game Boy (14.8 cm top to bottom), a tungsten dart (15 cm point to flight), 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 18 weeks?: Week 18 is in the second 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.

Stop doing this maths in your head

Once the counting turns into feeds, sleeps and nappies, BubSync keeps that record shared across everyone caring for your baby.

See how BubSync works All free tools