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How due dates are estimated
A full-term pregnancy is counted as 40 weeks — but, perhaps surprisingly, that count starts from the first day of your last menstrual period (LMP), about two weeks before conception actually happens. It's measured this way because the LMP is a date most people can pin down, while the exact moment of conception usually isn't known.
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last period to get your estimated due date.
How this calculator works
Enter the first day of your last period and your average cycle length. We apply Naegele's rule and then adjust for your cycle: if your cycle is longer than the typical 28 days you ovulate later, which pushes the due date back, and a shorter cycle brings it forward. We also estimate your likely conception date (around two weeks after your LMP) and show your current gestational age in weeks and days.
How accurate is it?
A due date is an estimate, not a deadline. Only about 4% of babies arrive on their exact due date, and a delivery any time from 37 to 42 weeks is considered full term. An early "dating" ultrasound — usually in the first trimester — is the most accurate way to confirm or adjust your due date, especially if your cycles are irregular.
The three trimesters
- First trimester: weeks 1–12.
- Second trimester: weeks 13–26.
- Third trimester: weeks 27–40.
This calculator shows which trimester you're currently in alongside your gestational age, so you can see at a glance how your pregnancy is progressing.
Helpful tools
Track your baby's development against your gestational age.
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Frequently asked questions
How accurate is a due date calculator?
It gives a solid estimate, but only about 4% of babies are born on the exact date. Anything from 37 to 42 weeks is full term, and an early ultrasound is more precise.
What if my cycles are irregular?
The LMP method is less reliable with irregular cycles. This tool adjusts for your average cycle length, but a dating ultrasound is the most accurate option for irregular cycles.
When is a pregnancy considered full term?
Between 37 and 42 weeks. Before 37 weeks is preterm; after 42 weeks is post-term, when your provider may discuss options.
Does this work if I conceived through IVF?
Not precisely. IVF and known-conception dates are calculated differently, so use the date your clinic provides rather than this LMP-based estimate.