From your last period
Enter the first day of your last menstrual period.
- Due date
- February 23, 2027
- Time left
- 15 weeks and 4 days
- Trimester
- 2nd
- Month
- 6
How the due date is calculated
Every count starts with the LMP, the first day of the last menstrual period, because the exact day of conception is rarely known. Doctors count pregnancy as if it began about two weeks before the egg was fertilized.
What if I do not remember my LMP?
Use the ultrasound tab. Enter the scan date and the gestational age from the report, and the calculator works backward to the matching LMP. In the first trimester, the size of the embryo is the most accurate way to date a pregnancy.
Due date from the conception date
Conception happens about 14 days after the LMP in a 28-day cycle. Subtract 14 days from the conception date to get the LMP. Example: conception on 03/29/2026 matches an LMP of 03/15/2026 and a due date of 12/20/2026, which is 266 days after conception.
When each trimester starts
| Trimester | Weeks | Months |
|---|---|---|
| 1st | 1 to 13 weeks, 6 days | Months 1 to 3 |
| 2nd | 14 to 27 weeks, 6 days | Months 4 to 6 |
| 3rd | 28 weeks to birth | Months 7 to 9 |
To see how weeks line up with months, use the weeks pregnant in months chart.
What counts as full term
A due date is a midpoint, not a deadline. Only about 4 in 100 babies arrive on the exact day. ACOG groups births by how far along the pregnancy is:
| Label | Gestational age |
|---|---|
| Early term | 37 weeks to 38 weeks, 6 days |
| Full term | 39 weeks to 40 weeks, 6 days |
| Late term | 41 weeks to 41 weeks, 6 days |
| Post-term | 42 weeks and beyond |
Baby size by week
Average figures for selected weeks. Every baby grows at a personal pace.
| Week | Length | Weight | Milestone |
|---|---|---|---|
| 8 | 0.6 in (1.6 cm) | 0.04 oz (1 g) | Heart beating, arms and legs forming |
| 12 | 2.1 in (5.4 cm) | 0.5 oz (14 g) | Organs formed, end of the 1st trimester |
| 16 | 4.6 in (11.6 cm) | 3.5 oz (100 g) | Movements get more coordinated |
| 20 | 10.1 in (25.6 cm) | 10.6 oz (300 g) | Halfway point, movement often felt |
| 24 | 11.8 in (30 cm) | 1.3 lb (600 g) | Lungs developing |
| 28 | 14.8 in (37.6 cm) | 2.2 lb (1 kg) | Start of the 3rd trimester |
| 32 | 16.7 in (42.4 cm) | 3.7 lb (1.7 kg) | Weight gain speeds up |
| 36 | 18.7 in (47.4 cm) | 5.7 lb (2.6 kg) | Settling into position for birth |
| 40 | 20.1 in (51 cm) | 7.5 lb (3.4 kg) | Full-term pregnancy |
Go deeper
Frequently asked questions
How do I calculate my due date?
Add 280 days, or 40 weeks, to the first day of your last period. An LMP of March 15, 2026 gives a due date of December 20, 2026. A cycle longer or shorter than 28 days shifts the date.
What week of pregnancy am I?
Count the days from the first day of your last period to today and divide by 7. The whole number is the week and the remainder is the days. For example, 171 days is 24 weeks and 3 days.
How do I calculate my due date from conception?
Add 266 days to the conception date, or subtract 14 days to find the LMP and add 280. Conception on March 29, 2026 points to a due date of December 20, 2026.
Which is more accurate, LMP or ultrasound dating?
An early ultrasound, in the first trimester, is usually the more accurate. LMP dating depends on memory and a regular 28-day cycle. If the two dates are far apart, your provider will often use the scan date.
When does each trimester start?
The 1st trimester runs through 13 weeks and 6 days, the 2nd runs from week 14 through 27 and 6 days, and the 3rd starts at week 28 and lasts until birth.
Can my due date change?
Yes. A first ultrasound can move the date by several days, and a due date is only an estimate. Most babies arrive within a few weeks on either side.
Sources and review
- American College of Obstetricians and Gynecologists (ACOG): Methods for Estimating the Due Date.
- American College of Obstetricians and Gynecologists (ACOG): definition of term pregnancy.