Pregnancy Weeks to Months Calculator
Turn weeks pregnant into months, or months into weeks, with a table for every week from 1 to 42.
The tool
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Weeks to months table, weeks 1 to 42
A full pregnancy is 40 weeks. In the table most sources use, that is 9 months, and week 20 is the start of month 5. By the calendar (days divided by 30.44) week 20 is about 4.6 months. Both are in the tables below.
Which weeks make up each month
| Month | Weeks | Weeks by the calendar |
|---|---|---|
| 1 | 1–4 | 4.3 |
| 2 | 5–8 | 8.7 |
| 3 | 9–13 | 13.0 |
| 4 | 14–17 | 17.4 |
| 5 | 18–22 | 21.7 |
| 6 | 23–27 | 26.1 |
| 7 | 28–31 | 30.4 |
| 8 | 32–35 | 34.9 |
| 9 | 36–40 | 39.1 |
| 9+ | 41–42 | — |
Every week from 1 to 42
| Week | Month of pregnancy | Calendar months | Trimester |
|---|---|---|---|
| 1 | 1 | 0.2 | First |
| 2 | 1 | 0.5 | First |
| 3 | 1 | 0.7 | First |
| 4 | 1 | 0.9 | First |
| 5 | 2 | 1.1 | First |
| 6 | 2 | 1.4 | First |
| 7 | 2 | 1.6 | First |
| 8 | 2 | 1.8 | First |
| 9 | 3 | 2.1 | First |
| 10 | 3 | 2.3 | First |
| 11 | 3 | 2.5 | First |
| 12 | 3 | 2.8 | First |
| 13 | 3 | 3.0 | First |
| 14 | 4 | 3.2 | Second |
| 15 | 4 | 3.4 | Second |
| 16 | 4 | 3.7 | Second |
| 17 | 4 | 3.9 | Second |
| 18 | 5 | 4.1 | Second |
| 19 | 5 | 4.4 | Second |
| 20 | 5 | 4.6 | Second |
| 21 | 5 | 4.8 | Second |
| 22 | 5 | 5.1 | Second |
| 23 | 6 | 5.3 | Second |
| 24 | 6 | 5.5 | Second |
| 25 | 6 | 5.7 | Second |
| 26 | 6 | 6.0 | Second |
| 27 | 6 | 6.2 | Second |
| 28 | 7 | 6.4 | Third |
| 29 | 7 | 6.7 | Third |
| 30 | 7 | 6.9 | Third |
| 31 | 7 | 7.1 | Third |
| 32 | 8 | 7.4 | Third |
| 33 | 8 | 7.6 | Third |
| 34 | 8 | 7.8 | Third |
| 35 | 8 | 8.0 | Third |
| 36 | 9 | 8.3 | Third |
| 37 | 9 | 8.5 | Third |
| 38 | 9 | 8.7 | Third |
| 39 | 9 | 9.0 | Third |
| 40 | 9 | 9.2 | Third |
| 41 | Past 9 | 9.4 | Third |
| 42 | Past 9 | 9.7 | Third |
Weeks 41 and 42 come after month 9. Calendar months are weeks times 7 divided by 30.44, to one decimal. Weeks are counted from the first day of your last period.
How this works
Choose how many weeks pregnant you are, and add days if you know them. The calculator shows which month of pregnancy that is, how many calendar months it equals, and which trimester you are in. To go the other way, switch to months to weeks and pick a month.
Weeks are counted from the first day of your last period, the way a midwife or doctor dates a pregnancy. That is why a full pregnancy is 40 weeks, even though the baby has been growing for about 38 of them.
Months are harder than weeks, because a month is not a fixed number of weeks. Most pregnancy sites, including What to Expect and The Bump, use one table: month 1 is weeks 1 to 4, month 2 is weeks 5 to 8, month 3 is weeks 9 to 13, and so on up to month 9 at weeks 36 to 40. Weeks 41 and 42 fall after month 9. This calculator uses that table, so its answer matches what you will read elsewhere.
Some people mean something simpler: how many calendar months have passed. For that, the calculator also gives the calendar figure, which is the number of days divided by 30.44, the average length of a month. At 20 weeks that is about 4.6 months, while the table calls it the start of month 5. Neither is wrong; they answer slightly different questions.
Under the calculator you will find a full table from week 1 to week 42, so you can look up any week without typing. For what is happening at your week, see the pregnancy week-by-week guide linked below.
The math behind it
Month of pregnancy comes from the completed week. Week 13 with 6 extra days is still week 13, so it is still month 3. The bands are: month 1 is weeks 1 to 4, month 2 is 5 to 8, month 3 is 9 to 13, month 4 is 14 to 17, month 5 is 18 to 22, month 6 is 23 to 27, month 7 is 28 to 31, month 8 is 32 to 35 and month 9 is 36 to 40.
Calendar months = (weeks × 7 + days) ÷ 30.44, rounded to one decimal. 30.44 is the average month in the Gregorian calendar (365.25 ÷ 12). Example: 20 weeks is 140 days, and 140 ÷ 30.44 is 4.6 months.
Months to weeks, by the calendar, is months × 30.44 ÷ 7. Five calendar months is about 21.7 weeks. The table version is simpler: month 5 covers weeks 18 to 22.
Trimesters follow the usual cut-offs: first is weeks 1 to 13, second is 14 to 27, third is 28 onward. The days to 40 weeks count back from 280 days.
In the ProbaBaby app
iPhone · iPad · Apple WatchKnow which week you are in without converting anything
The app counts the weeks for you from your due date and shows what each one means for you and your baby.
The app also
- Week-by-week content for weeks 1 to 42, with your baby's size and the WHO length and weight ranges each week.
- Your current week on a Home or Lock Screen widget, with a countdown, and on your Apple Watch.
- Twin pregnancies get their own note for most weeks, on the weekly card and in the timeline.
- A one-page Doctor Visit Summary PDF built around your current week, shareable by QR code (24 hours, one view).
Free to download and use: pregnancy, baby and mom tracking, 32 tools, PDF reports, widgets, Apple Watch and family sharing cost nothing. Only AI baby predictions use credits.
On the App Store
Sheet 1 of 1
Sample Doctor Visit Summary for a twin pregnancy at week 32: delivery timeline adjusted for twins, weight, kicks, contractions, symptoms, screenings and your questions on one sheet. Fictional patient.
Frequently asked questions
20 weeks pregnant is month 5 in the table most pregnancy sites use (month 5 is weeks 18 to 22). By the calendar it is about 4.6 months, because 140 days divided by 30.44 is 4.6.
A full pregnancy is 40 weeks. In the usual table that is 9 months, and by the calendar it is about 9.2 months. The "10 months" you sometimes hear counts 4-week months: 40 weeks divided by 4 is 10.
It varies from 4 to 5 weeks. In the table most sources use, months 1, 2, 4, 7 and 8 are 4 weeks each, and months 3, 5, 6 and 9 are 5 weeks each. The calendar average is 4.35 weeks a month.
12 weeks pregnant is month 3 in the usual table (weeks 9 to 13), and about 2.8 calendar months. You are still in the first trimester, which runs to week 13.
Sites choose between two methods: a week table like the one here, or dividing weeks by 4 or by 4.35. The answers can differ by about a month. Both are correct for what they measure. This calculator shows the table month and the calendar figure together so you can compare.
The first trimester is weeks 1 to 13, the second is weeks 14 to 27, and the third is week 28 until birth. The calculator shows your trimester next to the month.
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