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How Due Dates Are Estimated
Due dates are estimated using two common methods. The LMP method adds 280 days (40 weeks) to the first day of your last menstrual period, following Naegele's rule. The conception method adds 266 days (38 weeks) to the estimated conception date. Both methods assume a 28-day cycle and ovulation on day 14. Only about 5% of babies are born on their exact due date.
Understanding Pregnancy Weeks and Trimesters
Pregnancy is divided into three trimesters. The first trimester spans weeks 1-12 and involves critical organ development. The second trimester (weeks 13-26) brings visible growth and is often the most comfortable period. The third trimester (weeks 27-40) focuses on fetal weight gain and final development before birth.
Frequently Asked Questions
If you know the exact conception date, that method is more accurate since it eliminates the assumption about when ovulation occurred. However, most women know their LMP date more reliably. An early ultrasound (before 12 weeks) provides the most accurate dating.
Most women use the LMP method. If your cycles are irregular, or you are unsure of your LMP date, an early ultrasound is the best way to establish a due date. The dating ultrasound measures the embryo to estimate gestational age.
Due dates are estimates within a two-week window. Only 5% of babies arrive on their exact due date, and most births occur between 38 and 42 weeks. A baby is considered full term between 39 and 40 weeks.