Pregnancy Due Date Calculator

Due Date · Current Week · Milestones

Calculation Method

First day of last menstrual period

Most common method — adds 280 days (40 weeks)

Estimates only. Only your healthcare provider can confirm your due date via ultrasound. Always consult your OB/GYN or midwife.

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Enter your information above to find your due date and pregnancy milestones.

How Your Due Date is Calculated

The most common method is Naegele's Rule: take the first day of your last menstrual period (LMP), add 280 days (40 weeks). This assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, the due date is adjusted accordingly.

About 4% of babies are born exactly on their due date. Most arrive within two weeks before or after. The most accurate confirmation comes from a first-trimester ultrasound (before week 14), which can narrow the estimate to within 5 days.

The Three Trimesters

First Trimester (Weeks 1–13)

Major organ development. The embryo becomes a fetus at week 10. Miscarriage risk is highest in this period and drops significantly after week 12. Morning sickness typically peaks at weeks 8–10.

Second Trimester (Weeks 14–27)

Often the most comfortable period. The anatomy scan (18–20 weeks) reveals baby's gender and checks development. Baby movements become noticeable around weeks 18–22.

Third Trimester (Weeks 28–40)

Baby gains most of its weight. Lungs mature. Baby moves into position for delivery. Kick counts begin around week 28. Full term is defined as 39–40 completed weeks.

IVF Due Date Calculation

For IVF pregnancies, the due date is calculated from egg retrieval. 3-day transfer: add 263 days. 5-day blastocyst: add 261 days. This is more precise than LMP because fertilization date is known exactly.

This calculator provides estimates only. Your healthcare provider will confirm your official due date via ultrasound measurement. Always follow your OB/GYN's guidance.

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Frequently Asked Questions

How is my pregnancy due date calculated from my last period?+

The standard method is Naegele's Rule: take the first day of your last menstrual period, add 1 year, subtract 3 months, and add 7 days — which equals 280 days or 40 weeks. This assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, the calculator adjusts accordingly. About 4% of babies are born exactly on their due date — most arrive within 2 weeks before or after.

How accurate is an estimated due date from an LMP calculator?+

An LMP-based due date is an estimate with a margin of roughly 2 weeks. The most accurate method is a first-trimester ultrasound (before 14 weeks), which narrows the estimate to within 5 days by measuring the embryo directly. If the LMP and ultrasound dates differ by more than 5–7 days, your provider will typically update the official due date to match the ultrasound. Early ultrasounds are more accurate than later ones for dating purposes.

What are the three trimesters of pregnancy and how long is each?+

The first trimester runs from weeks 1–13 (roughly 3 months). The second trimester covers weeks 14–27, often the most comfortable period with reduced nausea and visible baby movements. The third trimester is weeks 28–40, when the baby gains most of its weight and prepares for birth. Full-term is defined as 39–40 weeks; preterm is before 37 weeks; late preterm is 34–36 weeks; early term is 37–38 weeks.

How do I calculate my due date if I had IVF?+

For IVF pregnancies, the due date is calculated from the egg retrieval date, not your last period. For a 3-day embryo transfer, add 263 days to the transfer date. For a 5-day blastocyst transfer, add 261 days. This is more precise than LMP-based calculation because the exact fertilization date is known. Your fertility clinic will confirm the official due date at your first OB appointment via ultrasound.

How many weeks is a full-term pregnancy?+

Full-term pregnancy is defined as 39 weeks 0 days through 40 weeks 6 days. "Early term" is 37–38 weeks; "full term" is 39–40 weeks; "late term" is 41 weeks; "post-term" is 42+ weeks. Babies born at 37–38 weeks are considered early-term and may need extra care. Delivery is typically induced or C-section scheduled at 41–42 weeks if labor has not started naturally, as post-term pregnancies carry increased risks.

What prenatal tests are recommended by trimester?+

First trimester: blood type and Rh factor, CBC, STI screening, first-trimester genetic screening (nuchal translucency ultrasound + blood test), and cell-free DNA testing (NIPT) at 10+ weeks. Second trimester: anatomy scan at 18–20 weeks, quad screen, gestational diabetes screening (glucose challenge at 24–28 weeks). Third trimester: Group B strep test at 35–37 weeks, repeat blood tests, and non-stress tests if indicated. Testing varies by age and risk factors.

Is it normal for my due date to change after an ultrasound?+

Yes — it is very common. If the ultrasound measurement differs from the LMP-based date by more than 5–7 days in the first trimester (or 10–14 days in the second trimester), your provider will typically update the official due date to match the ultrasound. This is more accurate and does not mean anything is wrong. Irregular cycles, uncertain LMP dates, or early conception timing all affect the LMP calculation.

How do I count pregnancy weeks — from conception or from my last period?+

Medically, pregnancy weeks are counted from the first day of your last menstrual period (LMP) — this is called gestational age. Fetal age (from actual conception) is about 2 weeks less than gestational age. When your doctor says you are "12 weeks pregnant," they mean 12 weeks gestational age. The reason for this convention is that LMP is usually known even when the exact conception date is not. All pregnancy milestones use gestational age.

Can I get pregnant with irregular periods?+

Yes, but irregular cycles make it harder to predict ovulation and calculate a due date from LMP. Ovulation tracking methods — basal body temperature charts, ovulation predictor kits (OPKs), or cycle-tracking apps — can help identify your fertile window. If cycles vary significantly (shorter than 21 days or longer than 35 days), consulting a gynecologist is advisable. An early ultrasound is especially important for accurate dating when periods are irregular.

What is the difference between gestational age and fetal age?+

Gestational age counts from the first day of your last menstrual period and is the standard medical measurement used for all prenatal care and developmental milestones. Fetal age (conceptional age) counts from actual fertilization — typically 2 weeks less than gestational age. When your provider says "week 20 anatomy scan," they mean 20 weeks gestational age, even though the embryo has only been developing for about 18 weeks. All this calculator's results use gestational age.