To convert pregnancy weeks to months, multiply the weeks by 7 and divide by 30.4, the average number of days in a calendar month. That is why a 40-week pregnancy is about 9.2 months, not exactly 9. The chart below gives the conversion for the weeks people ask about most.
Why pregnancy is counted in weeks
Pregnancy is counted from the first day of your last menstrual period, not from conception. Weeks are used because development changes quickly and calendar months have different lengths. In the first two weeks of this count you are not yet pregnant: ovulation and conception usually happen around week 2.
Weeks to months chart
| Weeks | Months (approx.) | Trimester |
|---|---|---|
| 4 | 0.9 | First |
| 8 | 1.8 | First |
| 12 | 2.8 | First |
| 13 | 3.0 | First |
| 16 | 3.7 | Second |
| 20 | 4.6 | Second |
| 24 | 5.5 | Second |
| 25 | 5.7 | Second |
| 28 | 6.4 | Third |
| 30 | 6.9 | Third |
| 32 | 7.4 | Third |
| 36 | 8.3 | Third |
| 40 | 9.2 | Third |
| 42 | 9.7 | Third |
Months are calculated as weeks × 7 ÷ 30.4. You will also see charts that group pregnancy into "month 1" (weeks 1 to 4), "month 2" (weeks 5 to 8) and so on. Both are approximations, which is why doctors and midwives stick to weeks.
The three trimesters
- First trimester: weeks 1 to 13.
- Second trimester: weeks 14 to 27.
- Third trimester: week 28 until birth.
Some health services start the second trimester at week 13, so you may see slightly different cut-offs.
What "term" means
Your due date is 40 weeks after the first day of your last period, but it is an estimate. Most babies are born in the weeks around it. Clinicians use these definitions:
- Early term: 37 weeks to 38 weeks and 6 days.
- Full term: 39 weeks to 40 weeks and 6 days.
- Late term: 41 weeks to 41 weeks and 6 days.
- Post-term: 42 weeks or more.
If an early ultrasound gives a different estimate, your healthcare team may adjust your due date. If your pregnancy came from IVF, the date is calculated from the embryo transfer instead: see our IVF due date guide.
Calculate your own dates
Use our weeks to months calculator for any week, or the due date calculator to see your estimated due date. Bloom Private is a cycle tracker, not a pregnancy app; after pregnancy, when your periods return, it can help you track your cycle again. Always follow the dates your healthcare team gives you.
Frequently Asked Questions
About 9.2 months. Calendar months are slightly longer than 4 weeks, so 40 weeks is a little more than 9 months.
About 5.7 months, in the second trimester.
Because the first day of the last period is a date most people know, while the exact day of conception is usually not known.