Doctors don't exactly "add" two weeks to the actual time the baby spends developing; rather, the commonly used gestational age calculation method begins counting pregnancy from an earlier point than conception.
Pregnancy is counted from the first day of the woman's last menstrual period (LMP), not the date of conception which generally occurs 2 weeks later. The unborn baby spends around 38 weeks in the uterus, but the average length of pregnancy, or gestation, is counted at 40 weeks. This practice exists for practical reasons:
Why count from the last menstrual period?
- Easier Recall: It's generally easier for a woman to remember the first day of her last menstrual period than to pinpoint the exact date of ovulation and conception.
- Standardized Calculation: This method provides a standardized way to estimate the due date, regardless of variations in individual menstrual cycles or ovulation timing.
Understanding Gestational Age
Gestational age is defined as the time elapsed since the first day of the woman’s last menstrual period. The chart below summarizes this calculation:
| Event | Approximate Time | Relationship to Gestational Age |
|---|---|---|
| First Day of Last Menstrual Period | Day 1 | Start of Gestational Age |
| Ovulation and Conception | About 2 weeks | Approximately 2 weeks later |
| Birth | About 40 weeks | Full Term Pregnancy |
Therefore, while the baby spends approximately 38 weeks developing, the gestational age is counted as 40 weeks because it includes the two weeks prior to conception. This is a convention for ease of calculation and standardization in prenatal care.
