How Far Along Am I? How Pregnancy Weeks and Due Dates Are Calculated
One of the first questions after a positive test is how far along you are and when the baby is due. The calculation is simple, though it has a quirk that surprises many people.
Pregnancy is counted from your last period
Pregnancy is measured from the first day of your last menstrual period (LMP), not from conception. Because ovulation usually happens about two weeks later, at “4 weeks pregnant” the embryo is really about two weeks old.
Doctors count this way because most people know the date of their last period, while the exact day of conception is rarely known.
Naegele’s rule
The estimated due date is 280 days (40 weeks) after the first day of your last period. A quick way to do it by hand:
- Add 7 days to the first day of your last period.
- Subtract 3 months.
- Add 1 year.
If your last period started on August 19, 2026: August 26, minus 3 months is May 26, plus a year gives May 26, 2027.
The rule assumes a 28-day cycle. With longer cycles, ovulation happens later and the due date moves back by the difference; with a 32-day cycle, about 4 days.
The three trimesters
| Trimester | Weeks | Highlights |
|---|---|---|
| First | 1 to 13 | Organs form; first ultrasound and screening tests |
| Second | 14 to 27 | Anatomy scan around week 20; you may start feeling movement |
| Third | 28 to birth | Growth and maturing; full term starts at 39 weeks |
Babies born between 37 and 38 weeks are considered early term, 39 to 40 weeks full term, and 41 weeks late term.
A due date is an estimate
Only a small share of babies arrive on their due date. Most are born somewhere between 37 and 42 weeks.
A first-trimester ultrasound is the most accurate way to date a pregnancy. Measuring the embryo can correct the date from your last period, especially if your cycles are irregular or you are unsure when your period started. When the two differ, providers usually go with the ultrasound date.
Weeks vs. months
Pregnancy weeks do not line up neatly with calendar months, which is why providers talk in weeks and days, such as “20w3d” for 20 weeks and 3 days. Roughly:
| Month | Weeks |
|---|---|
| 3rd | 9 to 13 |
| 6th | 22 to 27 |
| 9th | 36 to 40 |
This information is general. Your doctor or midwife will confirm your dates and guide your care.
Results are estimates for informational purposes only and are not financial, tax, legal or medical advice.