How many months is 4 weeks pregnant?

4 weeks is roughly 1 month. 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 4 weeks?

The NHS publishes around 0.2 cm long for week 4. Rather than one fruit, that length is offered here against eight rulers, each object carrying its own measurement: a hundred-and-thousand (0.2 cm across), a springtail (0.2 cm end to end), a pixel on a Game Boy screen (0.2 cm across), a stitch on a cricket ball (0.2 cm across one stitch), 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 4 weeks?

Week 4 is in the first 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.

4 weeks pregnant

What happens at 4 weeks pregnant?

At 4 weeks a baby is around 0.2 cm long, about the size of a hundred-and-thousand. That is roughly 1 month, in the first trimester, with about 36 weeks to go until 40.

Published length

0.2 cm long

In months

Roughly 1 month

Trimester

first · 36 weeks to go

What is developing at 4 weeks

The ball of cells has burrowed into the lining of the womb and split in two: one half becomes your baby, the other becomes the placenta. The amniotic sac is forming around it, and the yolk sac that feeds the embryo for the next few weeks is already in place. Nothing is recognisable as a baby yet — this is the fortnight of pure groundwork.

The size comparison above is a hundred-and-thousand, which measures 0.2 cm across. 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.

4 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 0.2 cm the NHS publishes for week 4. Pick whichever ruler you actually have a feel for. Why not fruit.

Sweet treats
a hundred-and-thousand
0.2 cm across
Australian animals
a springtail
0.2 cm end to end
The 90s
a pixel on a Game Boy screen
0.2 cm across
Sport
a stitch on a cricket ball
0.2 cm across one stitch
Things you already own
the head of a pin
0.2 cm across the head
Gaming
a pixel, back when you could see them
0.2 cm across
Fast food
a grain of salt off the chips
0.2 cm across
Toys
a Lego stud
0.48 cm across

Your dates, from 4 weeks

Set to someone who is exactly 4 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 4 weeks

How many months is 4 weeks pregnant?: 4 weeks is roughly 1 month. 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 4 weeks?: The NHS publishes around 0.2 cm long for week 4. Rather than one fruit, that length is offered here against eight rulers, each object carrying its own measurement: a hundred-and-thousand (0.2 cm across), a springtail (0.2 cm end to end), a pixel on a Game Boy screen (0.2 cm across), a stitch on a cricket ball (0.2 cm across one stitch), 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 4 weeks?: Week 4 is in the first 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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