Due Date Calculator

How this calculator works

Your estimated due date (EDD) is calculated using Naegele's rule, the method endorsed by the American College of Obstetricians and Gynecologists (ACOG).¹The formula adds 280 days — exactly 40 weeks — to the first day of your last menstrual period (LMP). This estimate assumes a 28-day cycle with ovulation on day 14.

If your average cycle is longer or shorter than 28 days, our calculator adjusts proportionally. For example, a 32-day cycle shifts the EDD four days later; a 24-day cycle shifts it four days earlier.

You can also calculate from a known conception or ovulation date (we add 266 days) or an IVF embryo transfer date (Day-5 adds 261 days; Day-3 adds 263 days), since the embryo's developmental age is precisely known in these cases.

How accurate is a due date?

Only about 4–5% of babies are born on their exact EDD. Research consistently shows that roughly 80% of births occur within 10 days of the calculated date, and approximately 95% within 20 days. The EDD is best understood as the midpoint of a distribution, not a precise prediction.

First-trimester ultrasound (performed before 14 weeks) is the most accurate method for confirming gestational age. Measurement of the crown-rump length (CRL) can date a pregnancy within ±5–7 days. If your ultrasound-derived date differs from your LMP-based date by more than 7 days in the first trimester, your provider will typically update your EDD to match the ultrasound.

After you get your due date: what to do next

Once you know your EDD, your next step is scheduling first prenatal care, ideally by 8–10 weeks of pregnancy. Early care establishes a baseline, screens for risk factors, and gives you access to first-trimester genetic screening (available weeks 10–13).

Key prenatal appointments to anticipate:

• First visit: ideally by weeks 8–10
• First-trimester screening: weeks 11–13 (nuchal translucency ultrasound + blood draw)
• Anatomy scan: weeks 18–22 (detailed fetal survey ultrasound)
• Glucose challenge test: weeks 24–28
• Group B Strep test: weeks 35–37
• Weekly visits: beginning at 36 weeks

Understanding pregnancy trimesters

Pregnancy is divided into three trimesters, each roughly 13 weeks long:

• First trimester (weeks 1–13): The embryo's major organ systems, neural tube, heart, and facial features form. Most miscarriages occur during this period. Nausea, fatigue, and breast tenderness are common early symptoms.
• Second trimester (weeks 14–27): Often called the "honeymoon trimester" — energy typically returns, morning sickness eases, and the baby grows rapidly. Fetal movement (quickening) is usually first felt around weeks 18–22.
• Third trimester (weeks 28–40+): Your baby gains most of their birth weight and the lungs mature in preparation for breathing. Prenatal visits become more frequent, and you'll discuss your birth plan with your provider.

A note about "full term"

The term "full term" specifically refers to weeks 39–40. ACOG defines:

• Early term: 37 weeks 0 days – 38 weeks 6 days
• Full term: 39 weeks 0 days – 40 weeks 6 days
• Late term: 41 weeks 0 days – 41 weeks 6 days
• Post-term: 42 weeks 0 days and beyond

Babies born at 39–40 weeks tend to have the best outcomes for lung maturity, feeding, and neurodevelopment. If your provider recommends a delivery date before 39 weeks without a medical indication, it is appropriate to ask why.