Fertilisation, pregnancy and birth

 Where does fertilisation take place?

When the male releases semen into the vagina, the spermatozoa swim through the cervix into the uterus. Usually, the ovum is fertilised in one of the Fallopian tubes. A spermatozoon and an ovum fuse together to form a zygote.

What happens during pregnancy?

For forty weeks, or three trimesters, the zygote develops and grows into a baby ready for birth. 

In the first trimester, the zygote implants in the wall of the the uterus. At this stage it is called an embryo. The placenta develops in the uterus. Nutrients and oxygen travel from it to the embryo along the umbilical cord. It also filters out toxins and removes carbon dioxide. The amniotic sac grows around the embryo. It is filled with amniotic fluid, which protects the growing embryo.

At the start of the second trimester, the embryo is between five and six centimetres long. It is now referred to as a foetus. The internal organs develop.

During the third trimester, the foetus grows and gains weight. Before birth, the foetus generally turns so that its head is pressing against the opening of the uterus.

Placenta

Amniotic sac

What happens during birth?

The process of birth starts when the muscles in the walls of the uterus begin to contract, causing the cervix to dilate or open. At this time the amniotic sac normally bursts. The contractions of the uterus push the foetus through the cervix and out of the mother’s body. 

Once the baby is born, its airways are cleared so that it can start to breathe. The umbilical cord is cut close to the baby’s skin. Contractions continue until the placenta is expelled from the mother’s body. Babies are born with an immature nervous system and digestive system.

 Generally they feed only on their mother’s milk for the first months of life.

PARA SABER MÁS

GEMELOS Y MELLIZOS

Puberty

 What happens to males during puberty?

When the testes and accessory glands are mature, the body can produce semen. Secondary sex characteristics also start to appear. 

• The voice changes and becomes deeper.

 • Hair appears on the face, in the pubic area and under the arms. 

• The body grows more quickly. The skeleton also becomes wider and muscles grow stronger. Males continue to grow in height for several more years.

What happens to females during puberty?

When ovaries are mature, they can produce fully developed ova. An ovum is released approximately every 28 days. This is called the menstrual cycle. Secondary sex characteristics also appear.

 • Breasts develop and grow, and hips become rounder. 

• Hair appears in the pubic area and under the arms. 

 • Females grow rapidly in height until puberty. They then continue to grow, but more slowly. Their muscles also grow stronger.

What happens during the menstrual cycle?

The menstrual cycle happens at regular intervals between the ages of 12 and 50 approximately. There are three stages to the menstrual cycle. 

1. Ovulation. Normally only one ovary releases an ovum. The ovum travels along one of the Fallopian tubes until it reaches the uterus. If the ovum is fertilised by a spermatozoon, it implants* in the wall of the uterus.

2. Preparation of the uterus. For about sixteen days after ovulation, the walls of the uterus thicken, growing a dense lining of blood vessels.

3. Menstruation. If the ovum is not fertilised, it does not implant in the uterus. After the lining has finished growing, it is eliminated from the body, along with some blood. This process lasts for three to six days and is commonly called ‘a period’.