Understanding Pregnancy Weeks and Trimesters
Pregnancy is counted in weeks starting from the first day of your last menstrual period (LMP), even though conception does not actually occur until about two weeks later. This convention, known as gestational age, is used by healthcare providers worldwide because the LMP date is usually known while the exact date of ovulation and conception is not. A full-term pregnancy is 40 weeks (280 days) from the LMP date, though healthy deliveries can occur anywhere from 37 to 42 weeks.
How the Due Date Is Calculated
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. This calculation assumes a regular 28-day cycle with ovulation on day 14. In practice, cycle lengths vary, and ultrasound measurements in the first trimester provide a more accurate due date estimate. This calculator uses Naegele's rule, which is the same formula used by most online due date tools and is the standard starting estimate before ultrasound confirmation.
The Three Trimesters
Pregnancy is divided into three trimesters, each lasting approximately 13 weeks. The first trimester covers weeks 1–13. This is the period of most rapid development: the embryo's organs form, and the risk of miscarriage is highest. Most people do not show outwardly, but morning sickness, fatigue, and breast tenderness are common. The second trimester covers weeks 14–26. Many consider this the most comfortable period — morning sickness typically fades, energy returns, and the baby bump becomes visible. The third trimester covers weeks 27–40. The baby gains weight rapidly, and the body prepares for delivery.
Estimated Conception Date
Conception typically occurs around 14 days after the first day of the last period for a standard 28-day cycle. The calculator estimates this by adding 14 days to the LMP. However, ovulation timing varies significantly between individuals and even between cycles, so the conception date is always an estimate. Intercourse within a 5-day window before ovulation or on the day of ovulation can result in conception, so the actual fertilization date could be earlier or later than estimated.
Gestational Age vs Fetal Age
Gestational age (what this calculator shows) counts from the LMP. Fetal age (also called embryonic age) counts from conception and is approximately 2 weeks less than gestational age. When your doctor says you are "12 weeks pregnant," they mean 12 weeks gestational age — your baby is actually about 10 weeks old in developmental terms. Ultrasound scans report gestational age to maintain consistency with other clinical measures.
This calculator is for informational purposes only and is not medical advice. Always confirm dates with your healthcare provider. Due dates are estimates — only about 5% of babies are born on their exact due date.