How this due date calculator works
The most common method is Naegele's rule: a full-term pregnancy lasts about 280 days (40 weeks) counted from the first day of your last menstrual period. Enter that date and the calculator adds 280 days, adjusting for your cycle length if it isn't the typical 28 days — a 32-day cycle, for example, means ovulation happens about four days later, so the due date moves four days later too.
If you know your conception date, the calculator counts 266 days forward. For IVF, it uses the embryo transfer date plus 261 days (5-day embryo) or 263 days (3-day embryo). Nothing you enter leaves your browser.
Gestational age and trimesters
Gestational age is counted from the start of your last period, not from conception, which is why pregnancy is described as 40 weeks even though conception happens about two weeks in. The first trimester runs through week 13, the second from week 14 to week 27, and the third from week 28 until birth.
Things to keep in mind
- A due date is an estimate. Only about 4% of babies are born on their due date, and most arrive between weeks 37 and 42.
- An early ultrasound can refine it. If it differs from your last-period date by more than about a week, your provider may adjust the due date.
- Use your real cycle length if it's irregular or clearly longer or shorter than 28 days.
- Plan ahead for leave. Once you know your due date, check your employer's parental leave policy and how much PTO you'll have by then.
Frequently asked questions
How is my due date calculated?
From the first day of your last menstrual period, add 280 days (40 weeks). The calculator adjusts this if your cycle is longer or shorter than 28 days, and uses different offsets for conception dates and IVF transfers.
How accurate is a due date?
It's a well-informed estimate rather than a precise prediction. Only around 4% of babies arrive on the exact due date; most are born within two weeks before or after it.
What if my cycles are irregular?
Enter your average cycle length if you know it. If your cycles vary a lot or you're unsure of your last period, an early ultrasound gives the most reliable dating.
What does 12 weeks and 3 days mean?
It's your gestational age — 12 full weeks plus 3 days since the first day of your last period. Providers use weeks and days to track pregnancy milestones.
Is this a substitute for medical advice?
No. This calculator is for general information only. Your doctor or midwife will confirm your due date and monitor your pregnancy.
This calculator is for general informational purposes only and is not medical advice. Consult your healthcare provider about your pregnancy.