First trimester

13 weeks pregnant: what is happening this week

Week 13 of 42worked example

7.4 cm

Measured: head to bottomabout the size of a peach
This week runs: Thu, Mar 26, 2026 to Wed, Apr 1, 2026
Days to the estimated due date: 196
Term window (37-42 weeks): Thu, Sep 17, 2026 to Thu, Oct 22, 2026

The NHS publishes this figure for week 13, about the size of a peach. It is an average from patient guidance rather than a measurement of any particular pregnancy, and the measurement basis changes at week 20 - see the note below the dates.

7.4 cm, peach-sized, and the last week of the first trimester on the convention this site uses. Ovaries or testes are fully developed by now. Movement has become deliberate rather than jerky - still far too small to feel, but different in kind from the reflexive twitching of a fortnight ago. Some foetuses start sucking a thumb around this point, which develops the reflex that will matter for feeding. Trimester boundaries deserve a note, because they are tidier in writing than in reality. There is no single authority that fixes them to the day. Thirteen weeks and six days, or fourteen weeks even, or the end of week twelve - all are used, and the differences are conventions rather than findings about what is happening inside. This site uses 0 to 13 weeks 6 days, 14 to 27 weeks 6 days, and 28 onwards, and labels it as a convention wherever it appears. What is real is the trend most people report across this fortnight: energy coming back, sickness receding, and the whole thing becoming easier to carry through an ordinary working week.

Put your own dates on week 13

Your estimated due dateNothing you type here leaves your browser. Without a date the panel shows a worked example built from a last period of Thu, Jan 1, 2026.

Week 1 began: Thu, Jan 1, 2026
Week 13 begins: Thu, Mar 26, 2026
Week 13 ends: Wed, Apr 1, 2026
Estimated due date: Thu, Oct 8, 2026

Dates run on ACOG's 280-day convention, the same arithmetic as the due date calculator. Share this week with your dates: /pregnancy/week-13?s=d:2026-10-08

Everything you type here is computed in your browser. Nothing you enter is stored on a server, sent anywhere, or shared — there is no account and no health data to leak.

What the NHS describes this week

The NHS puts the foetus at around 7.4 cm this week.
The ovaries or testes are fully developed.
Movements have become deliberate rather than jerky, and some foetuses begin sucking a thumb, which develops the sucking reflex used for feeding.

What happens around now

Trimester boundaries are a widely used convention rather than a finding. This site treats weeks 0 to 13 weeks 6 days as the first trimester, and says so wherever the label appears.

On the measurement. The NHS publishes a length for each week from 4 to 40. Up to week 19 it is measured head to bottom (crown to rump, the measurement a scan takes while the legs are curled). From week 20 it is measured head to heel, which is why the figure jumps from 15.3 cm at week 19 to 25.6 cm at week 20 - the baby did not grow 10 cm in a week, the ruler changed. The NHS states the change on its week 20 page.

Where week 13 comes from

NHS: week 13 of pregnancy
NHS, Week-by-week guide to pregnancy — the guide these weeks are built from

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