Complete Guide to Pregnancy Due Dates & Gestational Stages
Pregnancy lasts an average of 280 days (40 weeks) from the first day of the last menstrual period (LMP). Clinically, gestational age is measured from the LMP rather than the conception date because ovulation timing varies, whereas period start dates are more easily identifiable.
Naegele's Rule for Due Date:
Due Date = LMP Date + 1 Year − 3 Months + 7 Days (Adjusted for Cycle Length)
Due Date = LMP Date + 1 Year − 3 Months + 7 Days (Adjusted for Cycle Length)
The Three Trimesters Explained
- First Trimester (Weeks 1 – 13): Conception, blastocyst implantation, neural tube formation, and primary organogenesis.
- Second Trimester (Weeks 14 – 27): Often called the "golden trimester," nausea diminishes, fetal anatomy scans occur, and movement (quickening) is felt.
- Third Trimester (Weeks 28 – 40+): Rapid fetal weight gain, lung surfactant production, and body preparation for labor.
Frequently Asked Questions
Why does cycle length matter in due date calculation?
Naegele's standard rule assumes a 28-day cycle where ovulation happens around day 14. If your cycle is 32 days, ovulation occurred approximately 4 days later, extending your due date by 4 days.
When is a pregnancy considered full-term?
Early term is defined from 37 weeks 0 days through 38 weeks 6 days; full term is between 39 weeks 0 days and 40 weeks 6 days.