There are two common systems for denoting a fetus’s prenatal age. All of the prenatal ages on this website will be referenced from the time of conception for clarity. However, most obstetricians use an older system, called gestational age, which starts at the beginning of a woman’s last menstrual period. A woman’s period and the week afterwards actually gets counted towards her baby’s gestational age, even though she was not pregnant at that time.
Pregnancy starts at conception. Conception occurs after a woman ovulates, which is usually about two weeks before she will have her next menstrual period if she does not become pregnant. For a woman with regular, 28-day cycles, this is also about 2 weeks after her last menstrual period. If a woman knows exactly when she conceived, then she can calculate the baby’s gestational age by adding two weeks to the post-conception age. So, if a doctor tells a woman that she is 6 weeks pregnant, then the unborn child has been inside her for 4 weeks. Similarly, if a woman gives birth when she is 40 weeks pregnant, the baby has only existed for 38 weeks.
