Understanding Your Due Date
The estimated due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method is known as Naegele's Rule and assumes a 28-day cycle with ovulation occurring on day 14.
While this method provides a reliable estimate, only about 4% of babies are born on their exact due date. Most births occur between 38 and 42 weeks of pregnancy.
How Trimesters are Calculated
Pregnancy is divided into three trimesters, each marking specific developmental milestones:
- First Trimester (Week 1–12): The most critical period for organ development. Symptoms like morning sickness and fatigue are common.
- Second Trimester (Week 13–26): Often called the "golden period" of pregnancy. The baby's features become more defined, and you might feel movement for the first time.
- Third Trimester (Week 27–40): The final stage involves rapid growth and maturation of the lungs and brain. The baby prepares for birth.
Cycle Length Variations
If your menstrual cycle is longer than 28 days, your ovulation and conception likely occurred later than day 14. This calculator adjusts the due date forward by adding the difference between your cycle length and the standard 28 days.