Explore pregnancy week by week. This free pregnancy week calculator shows the trimester, month, progress and typical milestones for any week from 1 to 42.
Enter or slide to any week from 1 to 42 to see the trimester, the pregnancy month, how far along that is, and what’s typically happening. To work out your current week from a date, use the Pregnancy Calculator.
General educational information, not medical advice. For care and anything specific to your pregnancy, speak with your midwife or doctor.
How the Pregnancy Week Calculator Works
This tool works the other way round from most pregnancy calculators: instead of starting from a date, you enter a pregnancy week (1 to 42) and it shows the trimester, the pregnancy “month,” how far that is through a 40-week pregnancy, and what’s typically happening around that week.
Formula: Trimester = First if week is 13 or less, Second if week is 14 to 27, Third if week is 28 or more. Pregnancy month is looked up from fixed week ranges: weeks 1–4 = Month 1, 5–8 = Month 2, 9–13 = Month 3, 14–17 = Month 4, 18–22 = Month 5, 23–27 = Month 6, 28–31 = Month 7, 32–35 = Month 8, 36–42 = Month 9. Weeks remaining to 40 = 40 − week (or “past 40” once week exceeds 40). Progress = week ÷ 40 × 100%, capped at 100%.
- Pregnancy week — a whole number from 1 to 42 that you already know (or set with the slider), representing how many weeks pregnant you are.
Example Scenarios
| Week entered | Trimester | Pregnancy month | Weeks to 40 | Progress |
|---|---|---|---|---|
| 4 | First | Month 1 | 36 weeks | 10% |
| 12 | First | Month 3 | 28 weeks | 30% |
| 20 | Second | Month 5 | 20 weeks | 50% |
| 28 | Third | Month 7 | 12 weeks | 70% |
| 37 | Third | Month 9 | 3 weeks | 93% |
| 41 | Third | Month 9 | past 40 | 100% |
Pregnancy Week Calculator FAQ
Is this the same as the Pregnancy Calculator?
No. This calculator starts from a week number you already know and shows general information for that week. If you want to work out your current week from a date — your last period, due date, or conception date — use the separate Pregnancy Calculator instead.How does a week number turn into a trimester?
Weeks 1 through 13 fall in the first trimester, weeks 14 through 27 in the second, and week 28 onward in the third. These are the conventional trimester boundaries used across most pregnancy tracking, not a precise biological cutoff.Why do the “pregnancy months” not line up evenly with weeks?
A full-term pregnancy is measured as 40 weeks, which doesn’t divide evenly into calendar months, so the 9 “pregnancy months” shown here are approximate week-range bands rather than exact 4-week blocks — the last one, Month 9, spans 36 to 42 weeks.What’s typically happening around week 20?
Week 20 usually falls in the window for a mid-pregnancy anatomy scan (commonly offered around weeks 18–20), and many people start noticing early fetal movements around this stage. Your own timeline and scan schedule should come from your midwife or doctor.Why does week 37 mark “full term”?
By convention, a pregnancy reaching 37 weeks is considered full term, meaning the baby could arrive at any point from there without being classed as premature. This calculator reflects that as general information, not a prediction of when any specific baby will arrive.What happens if I enter a week past 40, like 41 or 42?
The calculator still shows the third trimester and Month 9, marks “weeks to 40” as past due, and caps the progress percentage at 100%. Going past 40 weeks is common and your care provider will typically monitor you and the baby more closely during this time.Related Calculators
If you don’t yet know your current week and only have a date to go on, the pregnancy calculator for working out weeks from your last period or due date can find it for you. For more on what’s developing at a given stage, see the baby development week-by-week calculator, and for tracking your own body through pregnancy, the pregnancy weight gain calculator is a useful companion.