Pregnancy Due Date Calculator

How Obstetric Due Dates are Calculated: Naegele's Rule

Human gestation averages approximately 280 days (40 weeks) from the first day of the last menstrual period (LMP). In clinical obstetrics, gestational age is conventionally dated from the LMP rather than conception because ovulation and fertilization occur roughly 14 days later and cannot be precisely pinpointed without ultrasound tracking.

The Mathematical Gestational Formula

Developed by German obstetrician Franz Karl Naegele, Naegele's rule calculates estimated date of delivery (EDD) using the calendar formula:

Step-by-Step Example Calculation

Suppose the first day of your LMP was January 1, 2026, with a standard 28-day cycle:

1. Add 1 year to LMP: January 1, 2027.
2. Subtract 3 months: October 1, 2026.
3. Add 7 days: October 8, 2026.
4. Since the cycle length is 28 days, cycle variance is 0 days.
5. Estimated Delivery Date = October 8, 2026.

If your cycle length is 32 days (4 days longer than average), ovulation likely occurred 4 days later, shifting the estimated due date forward by 4 days to October 12, 2026.

The Three Trimesters of Pregnancy

• First Trimester (Weeks 1 – 12): Rapid organogenesis, formation of neural tube, heartbeat detection, and limb bud development.
• Second Trimester (Weeks 13 – 26): Skeletal ossification, quickening (fetal movement felt by mother), and structural ultrasound anatomy scan (typically at 18-20 weeks).
• Third Trimester (Weeks 27 – 40+): Massive pulmonary maturation, brain lipid accumulation, subcutaneous fat deposition, and descent into the pelvic cavity.

Frequently Asked Questions (FAQ)