Quick answer: With a last period on 15 June 2026 and a 28-day cycle, on 6 November 2026 you are 20 weeks 4 days pregnant (144 days), in the second trimester, with a due date of 22 March 2027.
How it's calculated
Pregnancy is dated from the first day of the last menstrual period (LMP), not from conception. Naegele's rule puts the due date 280 days (40 weeks) after the LMP. For cycles longer or shorter than 28 days the date moves by the difference: due date = LMP + 280 + (cycle length − 28).
How far along you are is the number of days from that adjusted LMP to the date you choose, written as completed weeks and days. Trimesters follow ACOG: the first runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks.
This is an estimate from dates alone. An ultrasound in the first trimester, which measures the baby's crown–rump length, is the most accurate way to date a pregnancy; if it differs from the LMP date by more than 5 to 7 days, your doctor will use the ultrasound date. A pregnancy is full term from 39 weeks, and birth anywhere from 37 to 42 weeks is common.
Worked examples
- LMP 15 June 2026, 28-day cycle, on 6 November 2026: 144 days = 20w 4d, due 22 March 2027
- LMP 1 March 2026, 28-day cycle: 1 Mar 2026 + 280 days = 6 December 2026
- LMP 1 March 2026, 32-day cycle: 1 Mar 2026 + 280 + 4 = 10 December 2026
Trimesters and term (ACOG)
| Stage | Weeks |
|---|---|
| First trimester | 0 – 13w 6d |
| Second trimester | 14w 0d – 27w 6d |
| Third trimester | 28w 0d – birth |
| Early term | 37w 0d – 38w 6d |
| Full term | 39w 0d – 40w 6d |
| Late term | 41w 0d – 41w 6d |
| Post-term | 42w 0d and later |
Frequently asked questions
Why does pregnancy count from my last period and not from conception?
The date of the last period is usually known, while the day of conception is not. So by convention a pregnancy starts at the LMP, which is about two weeks before ovulation in a 28-day cycle. At conception you are already counted as about 2 weeks pregnant.
My cycle is irregular. Is this accurate?
Less so. With irregular cycles ovulation can fall on very different days, so the LMP date may be out by a week or more. An early ultrasound gives a far more reliable date.
Which date counts if my scan gives a different due date?
The ultrasound date. ACOG recommends changing the due date when a first-trimester scan differs from the LMP date by more than 5 days before 9 weeks, or more than 7 days from 9 to 13 weeks 6 days.
How many weeks is a full-term pregnancy?
Full term is 39 weeks 0 days to 40 weeks 6 days. Early term starts at 37 weeks and late term at 41 weeks.
When does each trimester start?
The first trimester runs to the end of week 13, the second from week 14 to week 27, and the third from week 28 until birth.
How many months is 40 weeks?
About nine months. Months have different lengths, which is why doctors count pregnancy in weeks instead.