Pregnancy week calculator

This shows the week you are in, how far into the trimester you are and when it ends.

Your details

This week is

Week 21

you are 20 weeks 0 days, second trimester

In midwife notation20+0 (140 days)
Week of this trimester8
Trimester endsend of week 26, 1 Dec
Full term from (37 weeks)17 Feb
Due date10 Mar, 140 days to go
Expected week of childbirth7 Mar to 13 Mar
Tell your employer by28 Nov (end of the 15th week before)
MATB1 form from your midwifefrom 21 Oct
Your leave starts20 Dec, the earliest allowed; give at least 28 days' notice, by 22 Nov
After 39 weeks you would be backSun 19 September 2027, if your baby comes on time
Due around now20 week scan (18 to 21 weeks, from 7 Oct)

Apps and leaflets differ on whether "week 12" means 11 weeks done or 12. Here, week 21 means you have completed 20 weeks, which midwives write as 20+0. After your dating scan, go by the scan dates.

Bleeding, tummy pain, a bad headache, blurred vision, fluid leaking or your baby moving less: call your maternity unit or midwife straight away, any time of day. In an emergency call 999.

The working

Completed weeks and the current week

140 days after your last period is exactly 20 weeks completed, so you are in week 21: week 8 of the second trimester. The 20 week scan usually happens between 18 and 21 weeks.

We also show the midwife notation (20+0), when this trimester ends, the date you reach full term at 37 weeks, the days to your due date and the next step on the NHS antenatal timeline, so you know what is coming.

Week numbering

Why it confuses everyone

"20 weeks pregnant" means 20 weeks completed. Some apps call that week 20, others week 21. Midwives write it as 20+0, with the days after the plus sign, which is the clearest.

Second trimester

Often the easier one

Sickness often eases by 16 to 20 weeks
Most people feel movements by 16 to 24 weeks
The 20 week scan checks your baby's growth and organs
Ask about the whooping cough jab from 16 weeks

Work and maternity leave

The dates that matter

GOV.UK counts from the expected week of childbirth, the Sunday to Saturday week your due date falls in. Due on Wednesday 10 March 2027, that week starts Sunday 7 March, so you need to tell your employer by Saturday 28 November, the end of the 15th week before. Leave can start from 20 December, 11 weeks before, and your midwife can give you the MATB1 form from 21 October, 20 weeks before the due date. You can take up to 52 weeks of leave, and Statutory Maternity Pay runs for up to 39 weeks. If you are off sick with a pregnancy related illness in the 4 weeks before the due week, leave starts automatically.

Read with care

What this cannot tell you

Your scan dateGo by the dating scan once you have it.
MovementsNever wait to report reduced movements.
HealthWeeks alone cannot show how things are going.

Questions

Common questions

What week of pregnancy am I in?

One more than your completed weeks: at 20+3 you are in week 21.

How many weeks is each trimester?

Weeks 1 to 12, 13 to 26, and 27 to birth.

What does 20+3 mean?

20 weeks and 3 days pregnant.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Support when you need it

Contact your midwife or GP any time you are worried, and call NHS 111 if you cannot reach them. If a date or a result brings up difficult feelings, Tommy's and the Miscarriage Association offer kind, free support.