BCBetter Calculators

Pregnancy Week Calculator

Find out how many weeks pregnant you are and when your baby is due.

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Enter your values and click Calculate

How It Works

Gestational age is calculated from the first day of the last menstrual period (LMP), which is the universal obstetric standard. The formula is simple: weeks pregnant = days since LMP ÷ 7. The estimated due date (EDD) is 280 days (40 weeks) after the LMP. This is Naegele's rule — the same method used by doctors and dating ultrasounds. Days remaining = 280 − days since LMP. The trimester boundaries follow standard obstetric definitions: weeks 1–12 are the first trimester, weeks 13–26 are the second trimester, and weeks 27–40 are the third trimester. Only about 5% of babies are born exactly on their due date — most arrive within a two-week window on either side. Always confirm gestational age with your OB or midwife, as ultrasound dating is more accurate in early pregnancy.

Examples

10 Weeks Pregnant
A woman whose LMP was 70 days ago checking her progress.
Result: 10 weeks pregnant (first trimester). 210 days (30 weeks) until due date.
Second Trimester
A woman 20 weeks into her pregnancy (140 days since LMP).
Result: 20 weeks pregnant (second trimester). 140 days (20 weeks) until due date.
Third Trimester
A woman 35 weeks along (245 days since LMP).
Result: 35 weeks pregnant (third trimester). 35 days (5 weeks) until due date.

Frequently Asked Questions

Why is pregnancy calculated from the last period, not conception?
Conception typically occurs around day 14 of the menstrual cycle (at ovulation), but the exact day is rarely known. The first day of the last menstrual period is easy to identify and provides a consistent reference point. Using LMP adds approximately two weeks to the gestational age compared to counting from conception — which is why a 'four-week pregnant' woman actually conceived only about two weeks ago.
How accurate is the estimated due date?
The EDD based on LMP is an estimate. Only about 5% of babies are born on their due date. Most births occur between 38 and 42 weeks. First-trimester ultrasound (between 8 and 13 weeks) is more accurate for dating because fetal size varies little at that stage. If your LMP date and ultrasound date differ by more than 7–10 days, your doctor will typically use the ultrasound date.
What are the trimester boundaries?
The first trimester covers weeks 1–12 and includes early fetal development, the highest risk of miscarriage, and common early symptoms like nausea and fatigue. The second trimester (weeks 13–26) is often called the 'honeymoon phase' — symptoms usually ease and the baby grows rapidly. The third trimester (weeks 27–40) is the final stretch, with the baby gaining weight and preparing for birth.
What if I don't know my LMP?
If your LMP is unknown or irregular, a dating ultrasound is the most reliable method to determine gestational age. An ultrasound measures the crown-rump length (in the first trimester) or head circumference, femur length, and abdominal circumference (in later trimesters) to estimate fetal age. Your healthcare provider can then calculate a revised due date based on the ultrasound measurements.

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