Pregnancy Due Date Calculator

A due date calculator based on Naegele's rule provides a clinically accepted estimate, but only about 4 to 5 percent of babies are born on their exact due date. The estimated due date represents the midpoint of a normal delivery window that spans roughly from 37 weeks (early term) to 42 weeks (post-term). Factors that affect accuracy include irregular menstrual cycles, uncertain last menstrual period dates, and individual variations in ovulation timing. An early ultrasound, typically performed between 8 and 12 weeks of gestation, can refine the estimate to within plus or minus five days. For the most precise dating, healthcare providers often combine LMP-based calculation with first-trimester ultrasound measurements of the crown-rump length.

Naegele's rule is a standard method used to estimate a pregnancy due date, developed by German obstetrician Franz Karl Naegele in the early 1800s. The rule calculates the estimated date of delivery by adding 280 days (40 weeks) to the first day of the last menstrual period, assuming a regular 28-day menstrual cycle. For cycles longer or shorter than 28 days, the calculation is adjusted by adding or subtracting the difference between the actual cycle length and 28 days. For example, if your cycle averages 32 days, four extra days are added to the standard 280-day calculation. The underlying assumption is that ovulation occurs approximately 14 days before the next period, and that fertilization happens shortly after ovulation.

The first trimester spans from week 1 through week 12 of pregnancy and is the period of the most dramatic developmental changes. During weeks 1 through 4, the fertilized egg implants in the uterine lining and the placenta begins forming. By week 5 or 6, the embryo's heart starts beating, which can often be detected via transvaginal ultrasound. By week 8, all major organ systems have begun to form, and the embryo is officially called a fetus. By the end of week 12, the fetus is about 3 inches long and has developed recognizable facial features, fingers, and toes. Common maternal symptoms during this period include morning sickness, fatigue, breast tenderness, mood changes, and frequent urination. The first trimester carries the highest risk of miscarriage, with most occurring before week 12.

Most healthcare providers recommend scheduling your first prenatal appointment between 6 and 8 weeks after the first day of your last menstrual period. During this initial visit, your doctor will confirm the pregnancy through a blood test or ultrasound, estimate the gestational age, check for a fetal heartbeat, review your medical history and current medications, and order baseline laboratory tests including blood type, Rh factor, and screening for infections. If you have a history of pregnancy complications, ectopic pregnancy, recurrent miscarriage, or chronic health conditions such as diabetes or hypertension, contact your provider sooner rather than waiting for a routine appointment. Seek immediate medical attention if you experience severe abdominal pain, heavy bleeding, or dizziness before your first appointment.

Going past your due date is a common occurrence and is not automatically a reason for concern. A pregnancy is classified as early term at 37 to 38 weeks, full term at 39 to 40 weeks, late term at 41 weeks, and post-term at 42 weeks and beyond. About 60 percent of women deliver on or before their due date, meaning roughly 40 percent go at least slightly past it. After 41 weeks, your healthcare provider will typically increase monitoring with twice-weekly non-stress tests, amniotic fluid index measurements, and biophysical profiles to ensure the baby remains healthy. Most providers will discuss and often recommend labor induction between 41 and 42 weeks because the risk of complications — including decreased placental efficiency, meconium aspiration, and stillbirth — rises after this point.

Pregnancy is measured from the first day of the last menstrual period and lasts approximately 40 weeks, or 280 days. It is divided into three trimesters. The first trimester covers weeks 1 through 12 and encompasses implantation, embryonic development, and the formation of all major organs. The second trimester spans weeks 13 through 26 and is characterized by rapid fetal growth, the development of movement patterns the mother can feel (known as quickening), and important screening tests such as the anatomy ultrasound. The third trimester runs from week 27 through delivery and involves significant weight gain for the baby, lung maturation, and the baby assuming a head-down position in preparation for birth. Each trimester has specific prenatal visits, screenings, and vaccinations recommended by medical guidelines.

Yes, completely. The Pregnancy Due Date Calculator runs entirely in your web browser using client-side JavaScript. No dates, health information, or personal data are transmitted to our servers or any third party. There are no accounts to create, no cookies storing personal health data, and no server-side processing. You can verify this by inspecting network requests in your browser's developer tools while using the tool. Given the sensitive and personal nature of pregnancy-related information, this privacy-first design is intentional. Your data stays on your device at all times, and the calculator works fully offline once the page has loaded.