malities too severe to maintain life.
the stages of prenatal development.
What major changes happen during the fetal stage of
When can the sex of a fetus be determined visually?
How is a fetus usually positioned in the uterus as birth nears?
nails; and the skull bones are largely ossiF ed. As
f gure 23.23
shows, the fetus is usually positioned upside down with its
head toward the cervix (
The birth of a live, healthy baby is against the odds, con-
sidering human development from the beginning. Of every
100 secondary oocytes exposed to sperm, eighty-four are fer-
tilized. Of these, sixty-nine implant in the uterus, forty-two
survive one week or longer, thirty-seven survive six weeks
or longer, and only thirty-one are born alive. Of those that
do not survive to birth, about half have chromosomal abnor-
Fused urogenital folds
Formation of external reproductive organs. (
) The genital tubercle, urogenital fold, and labioscrotal folds that appear
during the fourth week of development may di±
erentiate into (
) male external reproductive organs or (
) female external reproductive