How is the due date calculated?
This calculator uses Naegele's rule, the same method used by most healthcare providers. It adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle length differs from 28 days, the calculator adjusts accordingly. When using a conception date, it adds 266 days (38 weeks), since conception typically occurs about 2 weeks after the start of the last period.
How accurate is this estimate?
Only about 5% of babies are born on their exact due date. Most are born within 2 weeks before or after. The due date is an estimate meant to help you and your healthcare provider plan for prenatal care. An ultrasound in the first trimester can provide a more accurate estimate.
What are the three trimesters?
Pregnancy is divided into three trimesters, each about 13 weeks long. The first trimester(weeks 1–12) is when the baby's organs form and morning sickness is most common. The second trimester(weeks 13–26) is often called the "golden period" — energy returns and the baby grows significantly. The third trimester (weeks 27–40) involves rapid weight gain and preparation for birth.