when fetal milestones occur can be confushing the life of the fetus is typically measured from the time of ovulation or fertilization, but the length of a pregnancy is more commonly measured from the first day of the last menstrual period. because ovulation and fertilization take place about 2 weeks after the last menstrual period, the ovulation age of the fetus is alway 2 weeks less than the length of the pregnancy or the gestational age.