LIFE BEFORE BIRTH
The life of a human being begins at fertilization (or conception), when a sperm cell fuses with an oocyte (egg) to produce a new human organism. This individual is called a zygote at the one-cell stage of development, an embryo through the first eight weeks, and a fetus from eight weeks until birth. Each of us was once a zygote, embryo, and fetus, just as we were once infants, toddlers, and adolescents. All of these terms refer to stages in the life of a member of the species Homo sapiens.
Here are some of the milestones of human prenatal development. These dates are measured from the time of fertilization rather than from the last menstrual period (the dating method typically used during pregnancy), which occurs about two weeks earlier.
Conception: A new member of the human species begins. The zygote has a complete and unique set of 46 chromosomes (23 from each parent), the entire genetic blueprint. He or she needs only a suitable environment and nutrition in order to develop himself or herself through the different stages of human life.
6 days: The developing embryo, called a blastocyst at this stage, begins attaching to the wall of the mother’s uterus.
17 days: Blood cells have developed.
19 days: The eyes start to develop.
20 days: The foundation of the nervous system has been laid.
18-21 days: The heart begins to beat.
28 days: 40 pairs of muscles have developed along the trunk of the new individual; arms and legs are forming.
30 days: Regular blood flow exists within the vascular system; the ears and nasal passages have begun to develop.
6 weeks: The skeleton is complete and reflexes are present. The child has measurable brain waves.
7 weeks: The baby has the appearance of a tiny infant, with fingers, toes, and ears.
8 weeks: All organs are functioning—stomach, liver, kidney, brain—and all systems are intact.
9-10 weeks: The baby squints, swallows, and retracts his or her tongue.
11-12 weeks: The baby sucks his or her thumb and inhales/exhales amniotic fluid.
16 weeks: The mother may begin to feel her child’s movements. The baby grasps with hands, swims, kicks, and turns somersaults
18 weeks: The vocal cords are working.
20 weeks: A wealth of evidence indicates that, at least by 20 weeks, unborn children can experience pain. In the fifth and sixth months, the child responds to outside stimuli, including music and voices.
22 weeks: Babies today are usually capable of surviving outside the womb (with assistance) at 22 weeks post-conception (24 weeks from the last menstrual period). They can often survive even earlier. This is called viability.
38 weeks: The unborn child dramatically increases in size and weight during the second half of pregnancy until birth at about 38 weeks (40 weeks from the last menstrual period).
Birth, of course, is not the end of human development. The baby continues to grow and develop in the months, years, and decades to follow. The life of every human being is a continuum beginning at conception and proceeding (if all goes well) through the embryonic, fetal, infant, child, adolescent, and adult stages.
More detailed information about human prenatal development, including photographs and videos, is available from the Endowment for Human Development.