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.
both ovulation and gestational age are typically reporred in lunar months or in trimesters rather than in weeks.in lunar months, a total pregnancy is 10 months long; a fetus grows in utero for 9.5 lunar months or three full trimesters.
the following discussion of fetal developmental milestones is based on gestational weeks, because it is helpful when talking to expectant parents to correlate fetal development with the way they measure pregnancy from the first day of the last menstrual period, figure 9.6 illustrates the comparative size and apperance of human embryos and fetuses at different stages of development