How the due date is calculated
This uses Naegele's rule, the standard method used by clinicians worldwide:
Pregnancy is dated from the first day of your last menstrual period (LMP), not conception — which is why you're counted as "2 weeks pregnant" around the time you actually conceive. Your current week is simply the number of days since your LMP, divided by seven.
Your trimesters & milestones
- First trimester: weeks 1–13.
- Second trimester: weeks 14–27 (the anatomy scan is around week 20).
- Third trimester: weeks 28–40. "Full term" is 37 weeks.
Remember: only about 5% of babies arrive on their exact due date — most come within a week or two either side. A dating ultrasound gives the most accurate estimate, especially if your cycle is irregular.
How accurate is a due date from my last period?
Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter or irregular, a dating ultrasound will be more accurate — but the LMP method is a good first estimate.
How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by seven. This calculator does it for you and shows weeks + days.
Why is the baby's size shown as fruit?
It's the classic, friendly way to picture growth week by week — from a poppy seed early on to a watermelon near term. It's illustrative, not a medical measurement.
Can the due date change?
Yes. Your dating scan (usually 8–14 weeks) may adjust it, and that scan date becomes your official due date.
Is this private?
Yes — everything is calculated in your browser and nothing is stored or sent.
Evidence-based · last reviewed July 2026