What the pregnancy calculator does
The pregnancy calculator tells you how far along you are, which trimester you are in, your estimated due date and a timeline of key milestones. It works from whichever information you have: the first day of your last period, a due date your provider gave you, a known conception date, or an ultrasound with a measured gestational age. Whatever method you pick, all four agree on the same underlying 40-week timeline, so the results stay consistent.
Why pregnancy is counted from your last period
It seems odd to count from before conception, but the last menstrual period (LMP) is the date most people can pin down, while the exact day of ovulation and fertilisation usually is not known. By convention, a full-term pregnancy is 40 weeks (280 days) from the LMP, and conception happens around two weeks into that count. So at conception you are already considered "2 weeks pregnant", and the weeks shown here match what your midwife or doctor will use.
The four methods explained
- Last period: due date = LMP + 280 days, adjusted for your cycle length if it is not 28 days.
- Due date: if your provider gave you a date, everything is worked backward from it.
- Conception: due date = conception + 266 days (266 = 280 − 14), useful for IVF or known-ovulation pregnancies.
- Ultrasound: the scan's measured gestational age anchors the whole timeline to the scan date.
Understanding trimesters and milestones
The first trimester (weeks 1–13) covers the earliest development and often the toughest symptoms. The second (weeks 14–27) is frequently the most comfortable stretch, when many people first feel movement and have their anatomy scan around week 20. The third (week 28 to birth) brings rapid growth and preparation for delivery. The milestone table marks the end of the first trimester, the anatomy scan, the point of viability (around 24 weeks), full term (37 weeks) and the due date (40 weeks).
How accurate is it?
A due date is a best estimate, not a deadline. Just one baby in twenty arrives on the predicted day; the large majority are born anywhere from 37 to 42 weeks. If your cycles are irregular or your LMP is uncertain, an early dating ultrasound gives a more reliable date and will usually take precedence in your medical records.
Use with the other tools
To work specifically toward a due date, see the due date calculator. To estimate when conception happened, use the conception calculator, and for healthy weight gain in pregnancy, the pregnancy weight gain calculator.