To calculate how many weeks pregnant you are (also known as your gestational age), you need to know the date of your Last Menstrual Period (LMP). Starting from this date, count the weeks up to today's date. This number will correspond to the number of weeks pregnant you may be.

For example, if the current date was June 23, a woman who had her first period on April 15 should count of 9 weeks plus 6 days between the two dates.

The doctor can also tell you your most accurate gestational age by using an ultrasound. This date is considered to be more precise and helps to predict the probable date of delivery. Check out our online due date calculator to determine the your possible delivery date.

Online calculator

Use our calculator below to determine how many weeks pregnant you may be:

This calculator helps you find out the number of weeks and days that have passed since the start of your pregnancy simply by entering the date of your last period.

What if I don't know the date of my last period?

If you're not sure when your last period was, the most accurate way to find out how many weeks pregnant you are is to consult your obstetrician, who will be able to tell you your gestational age via ultrasound.

Weeks pregnant by trimester and month

This following table classifies your gestational age by weeks and months:

Trimester	Month	Weeks Included
1st trimester	1 month	up to 4 and a half weeks of pregnancy
1st trimester	2 months	4 and a half weeks to 9 weeks
1st trimester	3 months	10 to 13 and a half weeks of pregnancy
2nd trimester	4 months	13 and a half weeks to 18 weeks of pregnancy
2nd trimester	5 months	19 to 22 and a half weeks of pregnancy
2nd trimester	6 months	23 to 27 weeks of pregnancy
3rd trimester	7 months	28 to 31 and a half weeks of pregnancy
3rd trimester	8 months	32 to 36 weeks of pregnancy
3rd trimester	9 months	37 to 42 weeks of pregnancy

Usually a pregnancy lasts 40 weeks, but the baby can be born between 39 and 41 weeks without any concerns. If labor does not start spontaneously until the 41-week mark, the doctor may choose to induce labor with IV oxytocin.

How to calculate the estimated due date

To calculate your estimated due date, which should be around 40 weeks after your LMD, add 7 days to your LMD, and then count 3 months back and add one year.

For example, if the LMD was March 11, 2021, add 7 days, (March 18, 2021), then subtract 3 months (December 18, 2020) and add 1 year (December 18, 2021). In this case, the estimated due date is December 18, 2018.

This calculation is not 100% accurate, as the baby can be born between 37 and 42 weeks of pregnancy, however, it allows the parents and health care team to prepare for delivery.