Pregnancy due dates are typically calculated using Naegele's Rule, which estimates the expected date of delivery (EDD) based on the first day of your last menstrual period (LMP). The standard formula adds 280 days (40 weeks) to the LMP, which accounts for approximately 2 weeks before conception actually occurs.
The formula: EDD = LMP + 280 days. If your menstrual cycle is longer or shorter than the average 28 days, the calculator adjusts accordingly. For each day your cycle differs from 28, the due date shifts by one day. This adjustment accounts for the fact that ovulation typically occurs 14 days before the next period begins.
Three trimesters: The first trimester (weeks 1-12) is when the baby's major organs begin forming and morning sickness is common. The second trimester (weeks 13-26) is often called the "honeymoon phase" as energy returns and the baby's movements become noticeable. The third trimester (weeks 27-40) involves rapid growth and preparation for delivery.
Important milestones: Viability (around 24 weeks) is when the baby has a chance of survival outside the womb with medical support. At 28 weeks, the baby can typically breathe with assistance. Full term is considered 39-40 weeks. Only about 5% of babies are born on their exact due date — most arrive within two weeks before or after.
Medical disclaimer: This calculator provides estimates only. Your healthcare provider may adjust your due date based on ultrasound measurements, especially in the first trimester. Always follow your doctor's guidance for prenatal care and important medical decisions.