Pregnancy Calculator

About the Pregnancy Calculator

A pregnancy calculator determines your estimated due date (EDD), current gestational age in weeks and days, and key pregnancy milestones based on your last menstrual period (LMP) or conception date. Pregnancy is counted in weeks from the first day of your last menstrual period — not from conception — meaning a pregnancy is technically "2 weeks along" at the time of actual conception under standard obstetric counting. A full-term pregnancy is 40 weeks from LMP (280 days), though healthy deliveries occur anywhere from 37 to 42 weeks. Our free pregnancy calculator uses Naegele's Rule — the universally accepted obstetric formula for due date estimation — and provides a complete pregnancy timeline showing when each trimester begins, when major developmental milestones occur (heartbeat detectable at ~6 weeks, anatomy scan at ~20 weeks, viability at ~24 weeks), and all standard prenatal appointment windows. It also works in reverse: if you know your due date, it calculates your LMP and current gestational age. Compatible with single and twin pregnancy tracking, and aligned with American College of Obstetricians and Gynecologists (ACOG) gestational age standards.

How It Works

Naegele's Rule for due date calculation: add 280 days (40 weeks) to the first day of the last menstrual period. Equivalently: add 1 year, subtract 3 months, add 7 days from the LMP date. Example: LMP = July 10, 2025. Due date = July 10 + 280 days = April 15, 2026. Trimester divisions: First trimester = weeks 1-13 (conception through week 13). Second trimester = weeks 14-27. Third trimester = weeks 28-40+. Gestational age at today's date = (today's date - LMP) / 7 = weeks and fractional days. Due date accuracy: Naegele's Rule assumes a 28-day menstrual cycle with ovulation on day 14. Women with longer or shorter cycles may have more accurate dating from an early ultrasound (most accurate at 8-13 weeks, within 5 days accuracy).

Tips & Best Practices

• ✓Ultrasound dating is more accurate than LMP-based calculation, especially for women with irregular cycles. A first trimester ultrasound at 8-13 weeks can pinpoint gestational age within 3-5 days; second trimester ultrasounds are accurate to within 10-14 days.
• ✓Only about 5% of babies are born on their exact due date — the 40-week date is the midpoint of a normal delivery window. Full-term delivery is defined as 39-40 weeks; early term is 37-38 weeks; late term is 41 weeks; post-term is 42+ weeks.
• ✓Twins have a shorter average gestational length: fraternal twins typically deliver around 36-37 weeks; identical twins slightly earlier. Twin pregnancies are monitored more closely starting from the second trimester.
• ✓Key prenatal appointment windows: first appointment 8-10 weeks; nuchal translucency ultrasound 11-13 weeks; first trimester screening blood work 10-13 weeks; anatomy scan ultrasound 18-20 weeks; glucose challenge test 24-28 weeks; Group B Strep test 35-37 weeks.
• ✓Conception date versus LMP: actual fertilisation typically occurs 11-14 days after the first day of the LMP. If you know your conception date, add 266 days (38 weeks) to get the equivalent due date.
• ✓Fetal viability: the threshold of viability — when a premature baby has a chance of surviving with intensive medical care — is generally considered to be 24 weeks gestational age (22 weeks from conception).
• ✓Symptoms by trimester: first trimester nausea typically peaks at 8-10 weeks and resolves by 14-16 weeks for most women. Second trimester (weeks 14-27) is often called the "golden trimester" with increased energy and reduced nausea. Third trimester brings increasing physical discomfort as the baby grows rapidly.
• ✓Due date changes: if your 20-week anatomy scan shows a size discrepancy greater than 2 weeks from your LMP-based estimate, your provider may adjust your due date — this is common for women with irregular cycles or uncertain LMP dates.

Who Uses This Calculator

Newly pregnant women use the pregnancy calculator immediately upon receiving a positive test to understand their current gestational age, calculate their due date, and see the complete timeline of milestones ahead. Partners and family members use it to understand the progression of the pregnancy and the timing of important milestones and appointments. Obstetricians and midwives use gestational age calculations as the foundation of all prenatal care decisions — test timing, intervention windows, and delivery planning all depend on accurate gestational age. Fertility patients who underwent IVF use the calculator with their embryo transfer date instead of LMP, since the conception date is precisely known. People who experienced pregnancy loss use it to understand gestational age at the time of loss for medical records and support group discussions. Prenatal educators use the milestone timeline to teach anatomy of fetal development. Employers and HR departments use due date calculations to process maternity and paternity leave requests and plan coverage.