Understanding Due Dates and Gestational Age
Determining your estimated due date (EDD) is one of the first steps in tracking prenatal development. A normal pregnancy averages 40 weeks (280 days) from the first day of your last menstrual period (LMP). In clinical practice, gestational age is measured from the LMP rather than the date of conception, since the day of ovulation is often unknown.
How Naegele's Rule Works
Naegele's Rule is the standard method used to compute the EDD. It assumes a standard menstrual cycle of 28 days with ovulation occurring on day 14:
- Start with the first day of your Last Menstrual Period.
- Add 1 year.
- Subtract 3 calendar months.
- Add 7 days.
This is equivalent to adding 280 days to the LMP. For women with cycles longer or shorter than 28 days, the formula adds or subtracts the difference (e.g. for a 32-day cycle, add an extra 4 days).
The Trimesters of Pregnancy
Pregnancy is divided into three distinct phases or trimesters, marked by different stages of fetal development:
- First Trimester (Weeks 1 to 13): Period of rapid organogenesis, organ development, and extreme hormonal shifts.
- Second Trimester (Weeks 14 to 27): Period of growth, structural maturation, and initial fetal movements (quickening).
- Third Trimester (Weeks 28 to 40+): Final development phase, weight gain, lung maturation, and preparation for birth.