What is due date calculator?
A due date is an estimate of when a baby will be born. Doctors count pregnancy from the first day of the last menstrual period, which makes a full term pregnancy about 40 weeks.
This tool also lists key dates, such as the end of the first trimester and the start of full term.
How to use the due date calculator
- Choose the first day of your last period.
- Enter your usual cycle length if it is not 28 days.
- Read your due date, current week and milestones.
How the due date calculator works
Naegele's rule adds 280 days to the first day of the last period. If your cycle is longer or shorter than 28 days, the date is shifted by the difference. Weeks and days are counted from the first day of the last period to today.
Worked example
Common uses of the due date calculator
- Planning leave and appointments.
- Tracking pregnancy weeks.
- Sharing the estimated date with family.
- Preparing for each trimester.
- Discussing dates with a midwife.
Tips and common mistakes
An early ultrasound is more accurate than a calendar estimate. Your doctor may adjust the date.
Only a small share of babies arrive on the exact date. Most come within two weeks either side.
Background: what to know before you rely on it
Pregnancy is dated from the last menstrual period because the day of conception is rarely known. That means that in week 2 or 3 the baby has not been conceived yet, and the actual age of the baby is about two weeks less than the pregnancy age. The three trimesters run from week 1 to 13, week 14 to 26 and week 27 until birth. A baby born between weeks 39 and 40 is called full term, and one born before week 37 is called preterm.
Private, free and fast
This tool runs entirely in your browser. What you type or choose is not uploaded to a server and is not saved after you close the page, which keeps it fast and private. It is free to use as often as you like. See more in all date and time tools, or browse the full list of tools.
Frequently asked questions
How is the due date calculated?
By adding 280 days to the first day of the last menstrual period.
How accurate is it?
It is an estimate. Only about 4 percent of babies are born on the date.
What if my cycle is irregular?
Ask your doctor to confirm the date with an ultrasound.
Is this medical advice?
No. Always follow the guidance of your doctor or midwife.
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Results are for information and education. For legal, financial, medical or security decisions, check with a qualified professional. For legal deadlines, confirm how your contract or court counts days.