Pregnancy Week Calculator
Pregnancy week calculator: see how many weeks and days pregnant you are today, your trimester and month, your baby’s size and days to your due date.
Written by HealthSensibly Editorial TeamReviewed by the HealthSensibly editorial teamUpdated
Working out your current week
Weeks of pregnancy are counted from day one of your last period. That means during the first two weeks nothing has happened yet — you ovulate and conceive at around week 2. A full pregnancy runs for roughly 40 weeks. If a scan has already given you a due date, use that instead of your period date, as it tends to be more precise.
How weeks, months and trimesters line up
| Month | Weeks | Trimester |
|---|---|---|
| 1 | 1–4 | 1st (weeks 1–12) |
| 2 | 5–8 | |
| 3 | 9–13 | |
| 4 | 14–17 | 2nd (weeks 13–27) |
| 5 | 18–22 | |
| 6 | 23–27 | |
| 7 | 28–31 | 3rd (weeks 28–40) |
| 8 | 32–35 | |
| 9 | 36–40 |
Because a month is not a whole number of weeks, sources draw the month boundaries a little differently. Your doctor will always talk in weeks.
What you eat should change as you go — see our free trimester-by-trimester pregnancy diet plan.
Common questions
Can I work out my week by hand?
Yes. Take the number of days since your last period began and divide it by 7. For instance, 94 days gives 13 full weeks plus 3 days, so you are now in your 14th week.
Is the end of month three the same as 12 weeks?
Most charts place the end of month three at about 13 weeks, which is also close to the point where the first trimester finishes.
At what week do the later trimesters begin?
The second trimester begins at around week 13 and the third at week 28.
Should I use my period date or my scan date?
If you have had an early dating scan, its date is usually the more accurate one, so enter that due date in the calculator instead.
References: NHS: Pregnancy week by week · ACOG: Methods for estimating the due date
Checked against the sources listed above. This is general health information and not medical advice — please ask your doctor or a registered dietitian what suits you.
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