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Age of FetusA “full-term” human pregnancy is normally 259-294 days, or 37-42 weeks, with a modal length of 266 days. Fertilization age of the fetus uses the event of fertilization as time zero. Menstrual age uses the start of the mother’s last normal menstrual period (LNMP) as time zero, meaning that menstrual age is approximately two weeks older than fertilization age.Naegele's Rule states that pregnancy lasts 40 weeks or 280 days, which means add 9 months and 1 week from LNMP = due date OR subtract 3 months from last LNMP, and add a year and a week. However, pregnancy is really only 38 weeks or 266 days from fertilization. For a nice review of gestational age versus LNMP age, watch the Naegele.com video. After conception, the fertilized egg or conceptus is called a zygote. Rapid cell division of the zygote transforms it into a morula (meaning mulberry) within just 3 days. Then, a cavity forms within it to transform it into a blastocyst at 5 days. Implantation into the uterine lining occurs at the end of the first week of development. At the beginning of the third week, a process called gastrulation transforms the blastocyst into the embryo. This embryonic period last from weeks 3-8, and this is the critical period of development when the primordia of all the organ systems are established. At week 9, the conceptus enters the fetal period, which lasts until delivery at 38 weeks. The fetal period is characterized by differential growth of all of the primordia established in the embryonic period. Growth is tremendous during the fetal period, and differential growth among the various tissues and organ systems remodels the fetus to resemble the baby that is born at the end of gestation.
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