How to use the Pregnancy Due Date Calculator
- Choose how to calculate: from your last period, a known conception date or an IVF transfer.
- Enter the date. For the last-period method, adjust the cycle length if yours is not 28 days; for IVF, pick a day-5 or day-3 embryo.
- Read your estimated due date, how many weeks and days pregnant you are today and your trimester.
- Use the milestone table to see when common scans and tests usually happen. Past milestones are dimmed.
Features
- Three methods: last period, conception date and IVF transfer
- Adjusts for cycle lengths from 20 to 45 days
- Current gestational age in weeks and days, with trimester
- Countdown and progress bar to the due date
- Timeline of scans, tests and term milestones with dates
- Private: dates are processed only in your browser
How due dates are calculated
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception, because most people know when their period started but not the exact day they ovulated. Counted this way, a typical pregnancy lasts 280 days, or 40 weeks, even though the baby develops for about 38 of them.
From your last period
The classic method, known as Naegele's rule, adds 280 days to the first day of your last period. It assumes a 28-day cycle with ovulation around day 14. Longer or shorter cycles usually shift ovulation by the same amount, so the calculator adjusts: due date = LMP + 280 days + (cycle length − 28). With a 35-day cycle the due date moves 7 days later.
From a known conception date
If you know when you conceived, for example from ovulation tests, the due date is conception + 266 days (38 weeks). Weeks are then counted as if your period had started 14 days before conception, which is how pregnancies are dated in clinics.
From an IVF transfer
With IVF the embryo's age is known exactly. A day-5 embryo (blastocyst) is already five days past fertilization at transfer, so due date = transfer date + 261 days. For a day-3 embryo, due date = transfer date + 263 days. Both are 266 days minus the embryo's age.
Worked example
If your last period began on January 1, 2026 and your cycle is 28 days, your due date is January 1 + 280 days = October 8, 2026. On May 1, 2026 you would be 17 weeks and 1 day pregnant, in the second trimester.
Weeks, days and trimesters
Gestational age is written as weeks plus days, so "12w 3d" means 12 full weeks and 3 days. The trimesters here follow common obstetric practice:
| Stage | Gestational age |
|---|---|
| First trimester | Up to 13 weeks 6 days |
| Second trimester | 14 weeks 0 days to 27 weeks 6 days |
| Third trimester | 28 weeks 0 days until birth |
| Early term | 37w 0d to 38w 6d |
| Full term | 39w 0d to 40w 6d |
| Late term | 41w 0d to 41w 6d |
| Post term | 42w 0d and later |
How reliable is a due date?
A due date is an estimate, not a deadline. Only a small share of babies arrive on the exact day; most are born within a couple of weeks either side of it. An early ultrasound, especially a first-trimester scan that measures the baby's crown-rump length, is the most accurate way to date a pregnancy, and your provider may change your due date if the scan and your period dates disagree by more than a few days. IVF dates are the most precise of all, since the day of fertilization is known.
Using the milestone timeline
The table lists the usual windows for common checks, such as first-trimester screening, the mid-pregnancy anatomy scan, glucose screening for gestational diabetes and the group B strep test. Exact timing varies between countries and clinics, so treat the dates as a planning guide and confirm them with your midwife or doctor. To count down to the day, try the Days Between Dates calculator. Results from this calculator are estimates for general information and are not medical advice.
Frequently asked questions
How is my due date calculated?
From your last period, the calculator adds 280 days (40 weeks) to the first day of your last period and adjusts for your cycle length. From conception it adds 266 days, and from an IVF transfer it adds 266 days minus the embryo’s age.
What if my periods are irregular?
Enter your average cycle length for a rough estimate, but an early ultrasound is much more reliable when cycles vary. Your provider will usually date the pregnancy from that scan.
Why does pregnancy count two weeks before conception?
By convention, gestational age is counted from the first day of the last period, about two weeks before ovulation and conception. That is why you are already "4 weeks pregnant" around the time of a missed period.
How accurate is an IVF due date?
It is the most accurate type of estimate, because the date of fertilization is known exactly. Babies still arrive on a range of days around it, as with any pregnancy.
Can my due date change?
Yes. If an early ultrasound measures the baby as noticeably older or younger than your period dates suggest, your provider may move the due date to match the scan.