How weeks convert to months
Calendar months are not all the same length - they range from 28 to 31 days. To convert pregnancy weeks to months, this calculator uses the average Gregorian month of about 30.44 days (365.25 days divided by 12). That works out to roughly 4.3 weeks per month.
The conversion is always approximate, which is why obstetricians track pregnancy in weeks and days rather than months. Saying “20 weeks” is unambiguous; saying “4.5 months” is not.
Track it automatically
If you know your due date or the first day of your last period, the pregnancy week calculator tells you exactly how many weeks and days along you are right now - and updates every day without you having to recalculate.
If you need to find your due date first, start with the due date calculator.
Frequently asked questions
How many months is 20 weeks pregnant?
About 4.5 months, and the start of the second half of pregnancy. We convert using the average calendar month (about 4.3 weeks), so 20 weeks works out to roughly 4 months and 2 weeks.
Why is pregnancy 40 weeks but said to be 9 months?
Forty weeks is a little over 9 calendar months (and exactly 10 lunar months of 28 days). Because months vary in length, the week-to-month conversion is always approximate - which is why providers track pregnancy in weeks.
How are trimesters split?
The first trimester is weeks 1 to 13, the second is weeks 14 to 27, and the third is week 28 until birth. This calculator shows which trimester your weeks fall into.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.