How Is a Due Date Calculated?
The standard pregnancy due date formula is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Equivalently, add 7 days to your LMP date, subtract 3 months, and add 1 year.
For an LMP of March 1, 2024: add 7 days = March 8, subtract 3 months = December 8, add 1 year = December 8, 2024. Or simply: March 1 + 280 days = December 6, 2024 (minor rounding difference).
The formula assumes a 28-day cycle with ovulation on day 14. Women with cycles shorter than 28 days may deliver earlier than the calculated date; those with longer cycles may deliver later. Ultrasound measurements in the first trimester (7–13 weeks) are the most accurate way to confirm gestational age and refine the due date.
Trimester Breakdown and Key Milestones
Pregnancy is divided into three trimesters, each approximately 13 weeks:
First Trimester (Weeks 1–12): All major organs begin developing. By week 8, the embryo is considered a fetus with recognizable human features. Weeks 6–9 often involve nausea and fatigue. The first ultrasound typically occurs at 8–10 weeks. Risk of miscarriage is highest in this trimester (approximately 10–20% of known pregnancies).
Second Trimester (Weeks 13–26): Often called the 'honeymoon trimester' — most women experience reduced nausea and increased energy. Fetal movement typically first felt at 18–22 weeks (earlier with subsequent pregnancies). The anatomy scan ultrasound occurs at 18–20 weeks. The fetus reaches viability (potential to survive outside the womb with intensive medical care) at approximately 22–24 weeks.
Third Trimester (Weeks 27–40): Rapid fetal growth — the fetus gains about half a pound per week in this period. Lungs mature last, typically by 37 weeks. Full term is defined as 39–40 weeks; late preterm 34–36 weeks; early term 37–38 weeks. Only about 4% of babies are born on their exact due date; 90% arrive within 2 weeks before or after.