Fertilization and pregnancy

•Fertilization is the process whereby the sperm fuses with the ovum to form the zygote. It occurs in the upper part of the Fallopian tube.

•Only one sperm is needed to fertilize the ovum.

Fertilization and pregnancy

The journey of the sperms to the Fallopian tube

1. Sperms enter the vagina.

2. Sperms swim up the uterus

3. Sperms swim along the Fallopian tube

4. Sperms swim around the egg if it is present.

•The rapid growth and development of the baby between the zygote stage and birth is called pregnancy.

•The zygote undergoes cell division as it travels down the Fallopian tube towards the uterus.

•The zygote forms a ball of cells and is referred to as an embryo.

•About the seven days after fertilization, the embryo embeds in the tissues of the uterus lining. This process is called implantation.

Stages leading to implantation

1. Ovum fertilized by a sperm.

2. Zygote undergoes repeated cell divisions.

3. Embryo implants into the uterine wall.

•During this early stage of pregnancy, the embryo obtains its nourishment from nutrients in the ovum and the uterus lining.

•The surfaces of the embryo and the uterus lining bind firmly to form a large disc called the placenta.

•The job of nourishing the embryo is taken over by the placenta.

•By eight weeks after fertilization, the developing embryo becomes clearly human and is called the foetus.

Growth and development of the foetus in the uterus.

•At the placenta, the foetal and the maternal blood vessels flow very close to each other but do not mix.•The foetus is attached to the placenta by the umbilical cord.

•The umbilical cord has two blood vessels.a.An umbilical artery which carries carbon dioxide and other wastes from the foetus to the placentab.An umbilical vein which carries blood containing oxygen and food from the placenta to the foetus.

•Oxygen, soluble food, water and salts pass from the mother’s blood into the foetal blood.•Carbon dioxide and nitrogeneous waste pass from the foetal blood into the mother’s blood.•The foetus is completely surrounded by a membrane called amnion. The amnion is filled with amniotic fluid.•The amniotic fluid protects the embryo and the later the foetus by cushioning it from physical damage.•The time for the full development of a foetus is forty weeks. This is oftern reffered to as the gestation period.

•In humans, birth occurs in three main stages:

a.The expansion of the cervix so that the baby’s head can pass into the vagina.

b.The strong contractions of the uterus which push the baby out of the mother’s body.

c.Further contractions of the uterus to expel the umbilical cord and placenta from the mother’s body.