About the Pregnancy Calculator
A pregnancy calculator works out how many weeks pregnant you are, which trimester you are in, and the dates of key milestones. Gestational age drives every clinical decision in prenatal care, from screening timing to viability assessment.
The formula
Gestational age = days since LMP / 7Gestational age is counted in weeks from the first day of the last menstrual period, not from conception. Trimesters run weeks 1 to 13, 14 to 27, and 28 to 40.
How to use this calculator
- 1Enter your Last Menstrual Period (Year). The field starts at
2026, which you can overwrite. - 2Enter your Month (1–12). The field starts at
8, which you can overwrite. - 3Enter your Day. The field starts at
1, which you can overwrite. - 4Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Last Menstrual Period (Year) | 2026 |
| Month (1–12) | 8 |
| Day | 1 |
Result
Current: 1 weeks, 3 days
Trimester: 1st
Estimated Due Date: May 8, 2027
2nd Trimester Starts: Nov 6, 2026
3rd Trimester Starts: Feb 12, 2027
Days Until Due: 269
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
Dating from LMP rather than conception means gestational age runs about two weeks ahead of the embryo's actual age. This convention exists because the LMP date is knowable while the conception date generally is not, and all clinical guidelines are written in these terms.
Milestones are tied to specific weeks for good reason. First-trimester screening happens between 11 and 14 weeks, the anatomy scan around 18 to 22, glucose tolerance testing at 24 to 28, and viability improves dramatically between 24 and 28 weeks. Accurate dating is what makes these windows meaningful.
Things worth knowing
- An early ultrasound dates pregnancy more accurately than LMP, particularly with irregular cycles.
- Full term is 39 to 40 weeks. Anything from 37 to 42 weeks is within the normal range.
- Start prenatal care as early as possible — the first trimester is when screening options are widest.
- Folic acid supplementation matters most before and in early pregnancy, ideally beginning before conception.
- Report bleeding, severe pain, reduced fetal movement, or signs of pre-eclampsia to a clinician promptly.
Frequently asked questions
How many weeks pregnant am I?+
Count from the first day of your last menstrual period and divide by seven. This gestational age is about two weeks more than the time since conception, which is the standard clinical convention.
When do the trimesters change?+
The first trimester runs to the end of week 13, the second from 14 to 27, and the third from 28 to birth. Boundaries vary slightly between sources but these are the usual divisions.
When is a baby considered full term?+
39 to 40 weeks is full term. 37 to 38 is early term, 41 is late term, and beyond 42 is post-term. Babies born from 37 weeks are generally considered to have reached term.
Why is pregnancy dated from the last period?+
Because the date of the last period is usually known while the date of conception is not. It makes dating consistent and practical, at the cost of running two weeks ahead of actual embryonic age.