Pregnancy Calculator

Doctors don't typically date a pregnancy from conception, because most people don't know the exact day it happened — they date it from the first day of your last menstrual period (LMP) instead, then count forward. That's what this tool does: enter your LMP and it works out how many weeks and days along you are today, which trimester that puts you in, and a due date, using the same LMP-based counting convention used at a first prenatal visit.

It's worth being upfront about what this number is and isn't. A due date from LMP alone assumes a textbook 28-day cycle with ovulation on day 14, which isn't true for everyone — an early dating ultrasound is what actually refines it in real prenatal care. Treat the date here as a starting estimate, not a fixed appointment on the calendar.

People use this in the early weeks before a first ultrasound has confirmed anything, to get a rough sense of timing, or just to double check the math a midwife or app gave them. It's informational only, not a substitute for care from an OB or midwife who can confirm dates with an actual scan.

Because a last-period date is about as personal as data gets, everything here runs locally in your browser — nothing about your cycle, your due date, or the fact that you're even pregnant is transmitted anywhere or stored on a server.

The due date comes from Naegele's rule: add 280 days (40 weeks) to the first day of your last period. It's a clinical convention, not a precise prediction — only a small share of babies actually arrive on the exact calculated date.

Conception is estimated separately at LMP + 14 days, based on a typical mid-cycle ovulation. If your cycles run shorter or longer than 28 days, your actual ovulation — and true conception date — likely falls on a different day than this estimate.