Pregnancy Weeks to Months
Convert pregnancy weeks and days into a pregnancy month and a trimester.
Pregnancy months are not four-week blocks all the way through. Week 20 is month 5 on the chart most prenatal books use (weeks 18–22). Dividing 20 by 4.3 gives about 4.6 calendar months, which answers a different question.
Calculator Inputs
0 to 6. Seven days is another week.
Results
Enter values above to see your results
These dates are estimates for planning, not a medical diagnosis. An ultrasound is more accurate, and only a small share of babies are born on the exact due date.
How to use this calculator
- Enter the weeks from an ultrasound or from the week calculator, plus leftover days from 0 to 6. The example is 20 weeks and 0 days.
- Read “pregnancy month” for the chart used in week-by-week guides. Read “calendar months” if you want weeks divided by 4.345.
- The full chart is under the formula. Use it when you are converting several weeks, not just one.
How the calculation works
The pregnancy-month chart groups weeks into nine unequal months so that month 9 covers weeks 36–40, the usual window around a due date. Month 3 runs through week 13, which is why it is a week longer than month 2.
Calendar months are the continuous conversion: (weeks + days ÷ 7) ÷ 4.345. Twenty weeks is 20 ÷ 4.345 ≈ 4.6 calendar months. Both numbers can be true. They should not be mixed in the same sentence.
Trimester follows the week, not the month label. Week 20 is the second trimester. The third trimester starts at week 28, which is month 7 on this chart.
Worked example
20 weeks
Weeks 18–22= Month 520 ÷ 4.345= 4.6 monthsWeek 13 through 27= Second12 weeks and 3 days
12 + 3/7= 12.43 weeksWeeks 9–13= Month 3Before week 13= FirstPregnancy weeks to months
Chart months are the groups used in week-by-week pregnancy guides.
| Month | Weeks | Trimester |
|---|---|---|
| 1 | 1–4 | First |
| 2 | 5–8 | First |
| 3 | 9–13 | First, then second at week 13 |
| 4 | 14–17 | Second |
| 5 | 18–22 | Second |
| 6 | 23–27 | Second |
| 7 | 28–31 | Third |
| 8 | 32–35 | Third |
| 9 | 36–40 | Third |
Frequently Asked Questions
How many months is 20 weeks?
Month 5 on the pregnancy chart (weeks 18–22). In calendar months, 20 ÷ 4.345 is about 4.6 months.
How many months is 12 weeks?
Month 3, and still the first trimester. The second trimester starts at week 13.
Why not divide every month into 4 weeks?
Forty weeks is 10 groups of 4, but pregnancy is described as 9 months. The chart stretches some months to 4 or 5 weeks so month 9 lands on weeks 36–40.
Guides & Articles
How Many Weeks Pregnant Am I?
Divide days since the first day of the last period by 7. A worked example, why that count is about two weeks ahead of conception, and where months differ.
Pregnancy Trimester Dates Explained
First trimester through week 12, second from week 13, third from week 28. Calendar dates for a 1 January period, and how months overlap those boundaries.
Pregnancy Weeks to Months Chart
Pregnancy months are unequal groups of weeks, not weeks divided by four. The chart from month 1 through month 9, plus the calendar-month alternative.
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