How to use the Pregnancy Due Date Calculator
- Choose what you know: the first day of your last menstrual period (LMP), the conception date, or an IVF transfer date.
- Enter the date and, for the LMP method, your usual cycle length.
- Read the estimated due date, how many weeks pregnant you are today and upcoming milestones.
How it works
Obstetric dating counts pregnancy from the first day of the last menstrual period, not from conception. The standard rule (Naegele's rule) adds 280 days — 40 weeks — to the LMP:
Due date = LMP + 280 days + (cycle length − 28 days)From a known conception date the due date is 266 days later. For IVF, it is 263 days after a 3-day embryo transfer or 261 days after a 5-day (blastocyst) transfer.
Frequently asked questions
How accurate is a due date calculator?
It gives an estimate. Most babies are born between 37 and 42 weeks, and only around 4–5% arrive on the exact due date. A first-trimester ultrasound is the most accurate way to date a pregnancy.
Why is pregnancy counted from my last period?
The date of ovulation is usually unknown, while the start of the last period is easy to remember, so clinicians count from it. That is why you are considered about two weeks pregnant at conception.
What if my cycles are irregular?
Cycle-based estimates are less reliable with irregular cycles. Your healthcare provider will usually confirm the due date with an ultrasound.