Pregnancy Due Date Calculator

Enter the first day of your last period or the date of conception to estimate your due date, see how many weeks pregnant you are today, which trimester you are in, and the dates of key milestones.

First day of your last period

Usual cycle length

days

Cycle length is the days from the first day of one period to the first day of the next. Most cycles are 28 days, and the usual range is 21 to 35.

Pick the first day of your last period and your estimated due date shows here.

This is an estimate. Your doctor or midwife will confirm your due date, often with an early ultrasound, and may change it.

How a due date is worked out

Pregnancy is counted from the first day of your last menstrual period, so at conception you are already about two weeks pregnant. The most common method, Naegele's rule, adds 280 days (40 weeks) to that date. If your cycle is not 28 days long, the date moves by the difference, because ovulation happens later or earlier. If you know the conception date, the due date is 266 days after it.

The three trimesters

Trimester	Weeks
First	0 to 13 weeks
Second	14 to 27 weeks
Third	28 weeks until the birth

When is a baby full term?

Stage	Weeks and days
Early term	37 weeks 0 days to 38 weeks 6 days
Full term	39 weeks 0 days to 40 weeks 6 days
Late term	41 weeks 0 days to 41 weeks 6 days
Post-term	42 weeks and later

How reliable is the due date?

Only about 1 in 20 babies is born on the due date. Most arrive within two weeks either side.
Irregular cycles, uncertain dates and early ovulation all change the real timing.
An early ultrasound is often used to confirm or adjust the date, and your care provider's date is the one to use.
Contact your doctor or midwife about any concerns, and seek urgent help for severe pain, heavy bleeding or reduced movement.

Next steps

Once you have a date, book your first prenatal appointment and talk to your provider about screening, vitamins and your care options. This page is general information and cannot replace medical advice.

How to use the Pregnancy Due Date Calculator tool

Choose to start from your last period or from the conception date.
Pick the date. If you use your last period, also check your usual cycle length.
Read your estimated due date, how far along you are today and the trimester.
Look at the key dates list, then confirm your due date with your doctor or midwife.

Frequently asked questions

How is the due date calculated?

The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, the date is adjusted by the difference. From the conception date, the due date is 266 days later.

How accurate is a due date?

It is an estimate. Only about 1 in 20 babies is born on their due date, and most arrive within two weeks either side. Your doctor or midwife may move the date after an ultrasound.

What are the trimesters?

The first trimester is weeks 0 to 13, the second is weeks 14 to 27, and the third is week 28 until the birth. Doctors count weeks from the first day of the last period, so you are counted as about 2 weeks pregnant at conception.

What is full term?

Doctors call 37 weeks 0 days to 38 weeks 6 days early term, 39 weeks 0 days to 40 weeks 6 days full term, and 41 weeks 0 days to 41 weeks 6 days late term. A baby born before 37 weeks is preterm.

What if my cycles are irregular?

Then a last-period date may not give a reliable due date. Your care provider will usually use an early ultrasound measurement to date the pregnancy instead.