👶 Pregnancy Calculator
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception — which is why the “40 weeks” count starts about two weeks before an egg is even released. Add your LMP and average cycle length for an estimated due date.
Only about 1 in 20 babies arrives on the due date itself; think of it as the middle of a four-week window. Your doctor's ultrasound dating is more precise.
How to use this calculator
Enter the first day of your last menstrual period and your average cycle length to get your estimated due date, current week and trimester.
- Pick the First day of last period on the date field.
- Enter your Average cycle length in days, or leave the standard 28.
- Read your Estimated due date in the results.
- Check how far along you are, shown as week and day plus your current trimester.
Frequently asked questions
What does this pregnancy calculator compute and what are the inputs?
It works out how many weeks pregnant you are right now and which trimester you are in, starting from the first day of your last menstrual period (LMP) rather than from conception. Inputs are that LMP date and your average cycle length.
How does the calculation work?
It follows the standard obstetric rule: 280 days (40 weeks) counted from the first day of your last period, adjusted up or down if your cycle differs from 28 days. Weeks elapsed are counted from the same LMP date.
Can you show a worked example?
With a last period starting 2026-06-29 and a 28-day cycle, the estimated due date is 2027-04-05. By 2026-09-29 that is 92 days later, so about week 13, day 1, right at the start of the second trimester.
How accurate is the due date?
Only about one baby in twenty arrives exactly on the due date; most come within two weeks either side. An early ultrasound usually dates a pregnancy more precisely, so always confirm with your healthcare provider.