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Wondering when your little one will arrive? Our free Due Date Calculator provides a precise estimate based on your last menstrual period (LMP) or conception date.
When calculating a pregnancy due date, most people assume you simply add 9 months to the date of conception. However, the medical community uses a standardized algorithm called Naegele's Rule, named after the 19th-century German obstetrician Franz Karl Naegele.
Because the exact day of ovulation and conception is very difficult to pinpoint for most women, Naegele's rule bases the entire 40-week timeline on the very first day of your Last Menstrual Period (LMP). The formula is simple:
First Day of LMP + 7 Days - 3 Months + 1 Year = Estimated Due Date (EDD)
This means that for the first two weeks of your "pregnancy timeline," you weren't technically pregnant at all! Your body was just preparing for ovulation. When a doctor says you are 8 weeks pregnant, the fetus itself is actually only about 6 weeks old.
A standard 40-week pregnancy is broken down into three trimesters, each characterized by massive shifts in both maternal biology and fetal development.
The most critical phase for development. The embryo forms its neural tube (brain and spinal cord), heart, and major organs. For the mother, this is often characterized by intense fatigue, hormonal surges, and morning sickness.
Often called the "honeymoon phase." Morning sickness usually fades, energy returns, and the "bump" begins to show. The baby's sex can be determined via ultrasound, and the mother will feel the first flutter of fetal movement (quickening).
The rapid growth phase. The baby gains significant fat, the lungs finish maturing, and the brain develops rapidly. The mother will experience physical discomfort, Braxton Hicks contractions, and preparations for labor.
A calculator provides an excellent baseline, but your obstetrician may adjust your official EDD after your first trimester ultrasound (usually around 8 to 12 weeks).
During this early ultrasound, the technician measures the fetus from the top of the head to the bottom of the buttocks—known as the Crown-Rump Length (CRL). In the first trimester, embryonic growth is extremely uniform across all pregnancies. If the ultrasound CRL measurement differs from your LMP calculated date by more than 5 to 7 days, the doctor will legally change your due date to match the ultrasound, as it is considered the most clinically accurate dating method available.
Medical clarification on due date accuracy, preterm timelines, and IVF dating.