Almost every pregnancy tool starts from one number, the estimated due date, and every due date is an estimate: only about one baby in twenty arrives on the day. This guide shows how the date is worked out from different starting points, using the same arithmetic as the calculators. It is general information, not medical advice; your own midwife or doctor will confirm your dates, often with an early ultrasound.
Counting weeks from the last period
Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception, which happens about two weeks later. A full-term pregnancy is 40 weeks, or 280 days. So the due date is the LMP plus 280 days. For an LMP of January 10, 2026, the due date is October 17, 2026. A quick mental version, Naegele's rule, adds 7 days and 1 year, then subtracts 3 months. The Due Date Calculator and Pregnancy Calculator do this, and the latter also reports how far along you are: on 3 October 2026 that pregnancy is 38 weeks and 0 days (266 days) in.
Trimesters follow the weeks: the first runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks onward.
Other starting points
All of them land on the same 280-day clock:
| Known date | Rule | Example | Due date |
|---|---|---|---|
| Conception | + 266 days | January 24, 2026 | October 17, 2026 |
| Embryo transfer, day-5 | + 261 days | March 1, 2026 | November 17, 2026 |
| Embryo transfer, day-3 | + 263 days | March 1, 2026 | November 19, 2026 |
| Ultrasound at 8 weeks 3 days | + (280 − days of age) | March 1, 2026 | October 8, 2026 |
The 266 days come from conception being about 14 days after the LMP in a 28-day cycle (280 − 14). The Conception Calculator works backwards: from a due date of 17 October 2026 it centres conception on January 24, 2026 with a window of three days either side. A dating ultrasound is more accurate than a remembered LMP when the two disagree, which is why clinicians often adjust the date after it.
Ovulation, fertile days and your period
Ovulation happens about 14 days before the next period, not 14 days after the last one, so it moves with cycle length. With a 30-day cycle starting September 1, 2026, the next period is expected on October 1, 2026, ovulation on September 17, 2026 (day 16) and the fertile window, which spans the five days before ovulation and the day after, runs from September 12, 2026 to September 18, 2026. Use the Ovulation Calculator for your cycle and the Period Calculator to forecast the next several periods. Cycles vary month to month, so treat these as estimates rather than as contraception.
hCG in early pregnancy
The hormone hCG rises quickly in early pregnancy, typically doubling about every two to three days at first. The hCG Doubling Time Calculator works out the rate from two blood tests: levels of 1,000 and 1,800 mIU/mL taken 48 hours apart are a rise of 80% in 48 hours, a doubling time of about 56.6 hours. Single values and slower rises vary, and only a clinician can interpret them.
Weight gain
Recommended gain depends on weight before pregnancy. The US Institute of Medicine guidelines for a single baby are:
| Before-pregnancy BMI | Total gain (kg) | Total gain (lb) |
|---|---|---|
| Under 18.5 | 12.5 – 18 | 28 – 40 |
| 18.5 to 24.9 | 11.5 – 16 | 25 – 35 |
| 25 to 29.9 | 7 – 11.5 | 15 – 25 |
| 30 or more | 5 – 9 | 11 – 20 |
The Pregnancy Weight Gain Calculator converts these into a weekly target.
Common mistakes
- Counting from conception. Clinical dating counts from the LMP, so "38 weeks pregnant" means 36 weeks since conception.
- Assuming a 28-day cycle. Ovulation and the due date shift with longer or shorter cycles.
- Treating the due date as a deadline. Normal births occur from about 37 to 42 weeks.
- Self-interpreting hormone levels. Use the calculators for arithmetic, and your clinician for meaning.