About the Due Date Calculator
Doctors and midwives usually estimate the due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is known as Naegele’s rule, and it is how your due date is set at the first appointment before an early ultrasound refines it.
Pregnancy weeks are counted from the LMP, not from conception, so the day you actually conceived is usually around “2 weeks pregnant”. If your cycle is longer or shorter than 28 days, this calculator shifts the due date by the difference in ovulation day.
How to use
- Pick the first day of your last menstrual period.
- Enter your usual cycle length (days from the start of one period to the start of the next). If you aren’t sure, leave it at 28.
- Read your due date, the countdown, how far along you are and your trimester.
- Check the list of prenatal milestones below. The one that applies right now is highlighted.
How the due date is calculated
Due date = first day of last period + 280 days + (cycle length − 28 days)
A quick shortcut: take the first day of your last period, subtract 3 months, add 7 days and add one year. For example, an LMP of January 1, 2026 gives a due date of October 8, 2026. Only about 4–5% of babies are born on their due date; most arrive between 38 and 42 weeks.
Typical prenatal tests by week
| Weeks | What usually happens |
|---|---|
| 6–8 | First prenatal visit, dating ultrasound (heartbeat) |
| 11–13 | First-trimester screening: nuchal translucency (NT) scan and blood test (optional) |
| 18–22 | Anatomy scan (mid-pregnancy ultrasound) |
| 24–28 | Glucose screening for gestational diabetes |
| 36–37 | Late-pregnancy checks, e.g. baby’s position and, in some countries, a Group B strep test |
Schedules differ between countries, clinics and pregnancies. Always follow the plan from your own doctor or midwife.
Trimesters and term
The first trimester lasts until 13 weeks 6 days, the second trimester from 14 weeks to 27 weeks 6 days, and the third trimester from 28 weeks until birth. A baby born from 37 weeks is early term, 39–40 weeks is full term, 41 weeks is late term and 42 weeks or more is post-term. Before 37 weeks is preterm.
FAQ
My ultrasound due date is different. Which one is right?
An early ultrasound (especially before 14 weeks) that measures the baby’s crown-rump length is considered more accurate than the LMP date. If the difference is large, your provider will usually change your due date to match the ultrasound.
What if my periods are irregular?
With irregular cycles an LMP-based due date can be off by a week or more. Use this result as a rough guide and go by the due date from your ultrasound.
When do pregnancy weeks start?
The first day of your last period counts as 0 weeks 0 days. That’s why you are usually already 4–5 weeks pregnant when a home pregnancy test turns positive.
Can I calculate my due date from my conception date?
Yes. Add 266 days (38 weeks) to the conception date, or enter a period date 14 days before conception here with a 28-day cycle. For IVF, your clinic will give you a due date based on the embryo transfer date.