How many weeks pregnant am I?
Enter your due date or the first day of your last period. You will see your exact week and day, your trimester and a timeline of the dates worth knowing.
This tool is educational and does not replace your midwife or doctor. Nothing you enter leaves your device.
Pregnancy weeks, explained
A pregnancy is counted in completed weeks plus days from the first day of your last period. “12 weeks and 3 days” is written 12+3 on UK maternity notes and 12w3d or 12 3/7 elsewhere. You are “in week 13” at that point, which is why two people can describe the same day differently.
The dates on the timeline
| Point | When | Why it matters |
|---|---|---|
| Second trimester | 14 weeks 0 days | Start of the middle third. |
| First movements | 16 to 24 weeks | The NHS says you should start to feel your baby move in this window. |
| Third trimester | 28 weeks 0 days | Start of the final third. |
| Hospital bag packed | 37 weeks | The NHS suggests at least 3 weeks before the due date. |
| Early term | 37 weeks 0 days | Labour before this is considered preterm. |
| Full term | 39 weeks 0 days | ACOG's definition of full term begins. |
| Due date | 40 weeks 0 days | An estimate. About 1 in 20 babies arrive on the day. |
Next: use the due date calculator if you are starting from a conception or IVF date, or the weight gain calculator.
Questions, answered
How many weeks pregnant am I?
Count the days since the first day of your last period and divide by seven. If you know your due date instead, subtract 280 days from it to find the starting date. This calculator does both and shows weeks and days.
Why does pregnancy count from my last period and not conception?
Because the date of a period is usually known and the date of conception usually is not. It means that at “4 weeks pregnant” conception happened about two weeks ago.
When does each trimester start?
On the common convention, the second trimester starts at 14 weeks 0 days and the third at 28 weeks 0 days.
When will I feel my baby move?
The NHS says you should start to feel your baby move between 16 and 24 weeks of pregnancy. If you have not felt movement by 24 weeks, tell your midwife.
How many months is 20 weeks pregnant?
Twenty weeks is the halfway point of a 40-week pregnancy, a little over four and a half calendar months. Weeks are more precise than months, which is why midwives and doctors use them.
Sources
- NHS: Due date calculator
- NHS: Your baby's movements
- NHS: Pack your bag for labour
- ACOG Committee Opinion 579: Definition of Term Pregnancy
Sources last checked on 11 November 2026.