How is a Pregnancy Due Date Calculated?
The standard method for calculating a pregnancy due date is Naegele's Rule: add 280 days (40 weeks) to the first day of your Last Menstrual Period (LMP). This is used by most healthcare providers worldwide.
The Three Trimesters
- First Trimester (Weeks 1-12): Critical organ development. Morning sickness is common. The embryo becomes a fetus.
- Second Trimester (Weeks 13-27): Often called the 'golden trimester.' Energy returns, baby movements begin (quickening), and sex can usually be determined.
- Third Trimester (Weeks 28-40): Rapid fetal growth and preparation for delivery. Prenatal visits become more frequent.
Important Note
Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks before or after. Your OB or midwife may adjust your due date after an early ultrasound for more accuracy.