About the Pregnancy Weeks to Months Converter
This pregnancy weeks to months converter answers “how many months pregnant am I?” Enter how many weeks (and days) pregnant you are and it tells you which month of pregnancy you are in, your trimester, how many calendar months have actually passed and how many weeks remain until 40 weeks. Switch the direction to see which weeks belong to any pregnancy month.
Doctors and midwives count pregnancy in weeks, but friends, family and baby books often talk in months, and the two do not line up neatly because a month is about 4.3 weeks, not 4. This tool uses the common 9-month convention used by pregnancy guides (month 1 = weeks 0–4, month 5 = weeks 18–22, month 9 = weeks 36–40+), and the standard clinical trimesters: first trimester to 13 weeks 6 days, second from 14 to 27 weeks 6 days, third from 28 weeks.
Weeks are counted from the first day of your last period, so “20 weeks pregnant” means 20 completed weeks. A full-term pregnancy of 40 weeks is 280 days — about 9.2 calendar months, or exactly ten 4-week “lunar” months, which is why you may hear both “nine months” and “ten months” for the same pregnancy.
How to use the pregnancy weeks to months converter
- 1Choose whether to convert weeks to months or a month to weeks.
- 2Enter your completed weeks and days pregnant, or pick a pregnancy month.
- 3Read your pregnancy month, trimester and calendar months completed.
- 4Check the table to see how all nine months map to weeks.
Formula and method
The pregnancy month comes from a lookup table that splits 40 weeks into nine months of four or five weeks each, the convention used by most pregnancy books and apps. Trimesters follow the standard clinical split: the first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days and the third from 28 weeks to birth.
For an exact figure, total days of pregnancy are divided by 30.4375, the average calendar month length (365.25 ÷ 12), giving completed calendar months. Dividing by 28 gives 4-week (lunar) months — 40 weeks is exactly 10 of these. Time left is 280 days minus the days of pregnancy so far.
- weeks, days
- Completed gestational age from the last menstrual period
- 30.4375
- Average days in a calendar month
- 280
- Days in a 40-week pregnancy
Worked examples
20 weeks pregnant
Twenty weeks falls in the 18–22 week block, so you are in your 5th month and second trimester. 140 days ÷ 30.44 ≈ 4.6 calendar months have passed, exactly five 4-week months, with 20 weeks left to go.
30 weeks 4 days
At 30 weeks 4 days (214 days) you are in the 28–31 week block — the 7th month — and in the third trimester. 214 ÷ 30.44 ≈ 7.0 calendar months have passed and 280 − 214 = 66 days (9 weeks 3 days) remain.
How many weeks is 6 months pregnant?
The 6th month of pregnancy covers weeks 23 through 27, the end of the second trimester. The third trimester starts at 28 weeks, which is the beginning of the 7th month.
Frequently asked questions
How many months is 20 weeks pregnant?+
Twenty weeks is in the 5th month of pregnancy and is the halfway point of a 40-week pregnancy. About 4.6 calendar months have passed since the first day of your last period.
Is pregnancy 9 months or 10 months?+
Both, depending on how you count. A 40-week pregnancy is about 9.2 calendar months, but exactly ten 4-week (lunar) months. Most guides divide it into nine months of four to five weeks.
When does each trimester start?+
The standard clinical definition runs the first trimester up to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks until birth.
If I am 12 weeks pregnant, am I 3 months pregnant?+
You are in your 3rd month (weeks 9–13), but only about 2.8 calendar months have passed since your last period. You complete the third month at around 13 weeks.
Why do weeks not divide evenly into months?+
A calendar month averages 30.44 days, or about 4.35 weeks, so some pregnancy months contain four weeks and others five. That is why month charts vary slightly between sources.