Pregnancy Calculator
Enter the first day of your last menstrual period to estimate your due date, current week of pregnancy, and trimester — instantly.
This calculator provides an estimate for informational purposes only — not medical advice. Please confirm your pregnancy and due date with a healthcare professional.
How the due date is calculated
The calculator uses a cycle-length-adjusted Naegele's rule, the standard method doctors have used for over 200 years:
- Approx. birth date (due date) = first day of your last menstrual period (LMP) + 280 days + (your cycle length − 28). A 32-day cycle, for example, pushes the estimate 4 days later than the classic 28-day formula.
- Conception date (approx.) = LMP + (cycle length − 14) days. Ovulation happens roughly 14 days before your next period, so a longer cycle means a later ovulation — and a later conception.
- Earliest reliable pregnancy test = LMP + your cycle length — the first day of your missed period. Testing a week after that is even more accurate.
- Current gestational age = today minus your LMP, shown in weeks and days — the same way clinics count pregnancy weeks.
- Trimester = weeks 1–12 (first), 13–26 (second), 27–40 (third).
The classic rule assumes a regular 28-day cycle with ovulation around day 14. Adjusting for your real cycle length gives a fairer estimate — but your provider can refine the date further with an early ultrasound, which is the most accurate way to date a pregnancy.
Frequently asked questions
How accurate is Naegele's rule for the due date?
It gives a good estimate for regular 28-day cycles, but only about 5% of babies are born on their exact due date. Irregular cycles, unknown conception dates, and individual variation all shift the real date — most births happen within two weeks of the estimate.
What if I don't know the date of my last period?
Use your best estimate for a rough idea, then ask your doctor for a dating ultrasound. It measures the fetus directly and estimates gestational age more precisely than the calendar method.
When do the trimesters start and end?
The first trimester is weeks 1–12, the second is weeks 13–26, and the third is weeks 27–40. These boundaries are the ones most commonly used by healthcare providers.
Should I still see a doctor after using this calculator?
Yes. This calculator is an estimate for information only. A healthcare provider will confirm the pregnancy, refine the due date, and arrange the prenatal care you and the baby need.
How is the conception date calculated?
Conception usually happens around ovulation, which we estimate as your LMP plus (cycle length − 14) days — day 14 for a 28-day cycle, day 18 for a 32-day cycle. It is an estimate: the exact day cannot be known from dates alone.
When should I take a pregnancy test for a reliable result?
Home tests are most reliable from the first day of your missed period (your LMP plus your cycle length). Testing one week after a missed period is even more accurate. Testing earlier can give a false negative because hormone levels may still be too low.