The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation around day 14. If you know your conception date, add 266 days (38 weeks). Ultrasound dating in the first trimester can refine the due date with greater accuracy. Only about 4-5% of babies are born exactly on their due date—most arrive within two weeks before or after.

Pregnancy is dated from the first day of your LMP because it's a date most women can recall, whereas the exact day of conception is often uncertain. Weeks 1-2 of pregnancy are actually before ovulation and fertilization occur. This standardized method ensures consistency across healthcare providers worldwide. Gestational age reflects the age of the pregnancy, not the embryonic age of the baby.

The first trimester spans weeks 1 through 13, when major organs and structures form. The second trimester covers weeks 14 through 26, often called the "golden period" as many early symptoms ease and fetal movement becomes noticeable. The third trimester runs from week 27 through delivery (typically week 40), characterized by rapid fetal growth and preparation for birth. A pregnancy reaching 37 weeks is considered full-term.

Due dates are estimates—only about 4-5% of women deliver on their exact due date. Approximately 60% deliver within a week of their due date, and about 90% deliver within two weeks. First-trimester ultrasound dating can improve accuracy to within 5-7 days. Factors like cycle length variation, timing of ovulation, and individual differences all affect the actual delivery date.

If your cycles are irregular, the LMP-based calculation may be less accurate. In such cases, an early ultrasound (typically between 8-12 weeks) provides a much more reliable due date by measuring the crown-rump length of the embryo. Many healthcare providers will adjust your due date based on ultrasound findings. You can still use this calculator as a starting point, but discuss any discrepancies with your doctor.

Gestational age is measured from the first day of your LMP and is what healthcare providers use to date a pregnancy. Fetal age (or embryonic age) is measured from conception, which typically occurs about two weeks after LMP. So at 8 weeks gestational age, the baby is about 6 weeks from conception. This calculator primarily uses gestational age, which is the standard medical convention.

Most healthcare providers recommend scheduling your first prenatal visit around 8-10 weeks of pregnancy. This visit typically includes a full medical history review, physical exam, blood tests, and often a dating ultrasound. If you have risk factors, your doctor may want to see you earlier. Don't wait too long—early prenatal care is associated with better pregnancy outcomes.

The American College of Obstetricians and Gynecologists (ACOG) defines full-term as 39 weeks through 40 weeks and 6 days. Early-term is 37-38 weeks, full-term is 39-40 weeks, late-term is 41 weeks, and post-term is 42 weeks and beyond. Babies born at full-term generally have the best health outcomes, with lower risks of respiratory and other complications.

First trimester (weeks 11-14): Nuchal translucency ultrasound and blood tests for chromosomal screening. Second trimester (weeks 15-20): Quad screen blood test and detailed anatomy ultrasound around week 20. Third trimester (weeks 24-28): Glucose tolerance test for gestational diabetes, and Group B strep screening around weeks 35-37. Your provider may recommend additional tests based on your age and risk factors.

It's very common to go past your due date, especially for first-time mothers. Your healthcare provider will monitor you more closely after 40 weeks with non-stress tests and ultrasound to check amniotic fluid levels. If you reach 41-42 weeks without labor, your doctor may discuss induction options. Always stay in close communication with your healthcare team and report any concerning symptoms immediately.