Introduction

General concepts

Human embryology is the study of human development during pregnancy. It covers the period from fertilization up to birth. Human embryology is essential in understanding adult anatomy as it describes how the complex structures of the human body develop. The better our understanding of developmental processes, the more readily we understand the adult body.

Pregnancy is subdivided into two periods of development: the embryonic period extends from fertilization until the end of the 8th week of development; the fetal period extends from the beginning of the 9th week until birth. The average time from ovulation until birth is 264-270 days, or 38-38½ weeks.
In embryology, the start of pregnancy is calculated from the last ovulation as ovulation and fertilization occur very close together. In obstetrics, the start of pregnancy is counted from the first day of the last menstrual period (LMP). The time between the LMP and ovulation varies but on average the LMP occurs two weeks before ovulation. Taking the LMP as the starting point gives a pregnancy duration of 40 weeks. This time is subdivided into 10 lunar months (28 days per month).
Viability means a baby's capacity to survive outside the uterus. Due to progress in prenatal medicine, viability is becoming possible earlier within pregnancy. It is assumed nowadays that viability is possible at 20 weeks after ovulation, though the baby must weigh a minimum of 400 grams. Viability depends mainly on maturation of the baby's lungs.