From Fertilization to Childbirth

Pregnancy follows fertilization. The baby-to-be passes through multiple stages of development, including the zygote, the embryo, and the fetus. Birth occurs at the end of pregnancy.

Once fertilization occurs, pregnancy lasts an average of 38 weeks, which is just under 9 months. This period is often separated into 3 trimesters which are characterized by the key moments in the baby’s development.

Weeks 1 to 12 of pregnancy correspond to the first trimester. During the first trimester:

• the fertilized ovum (zygote) becomes a fetus approximately 9 cm long;

• the amniotic sac and the placenta develop;

• the organs start developing;

• fingers and toes form;

• the fetus starts moving.

Weeks 13 to 26 of pregnancy correspond to the second trimester. During the second trimester:

• the fetus grows on average from 9 cm to 30 cm long;

• the nervous system and the skeleton start developing;

• hair and nails start growing;

• the senses develop and sharpen.

Weeks 27 to 38 of pregnancy correspond to the third trimester. During the third trimester:

• the fetus grows on average from 30 cm to 50 cm long;

• all the organs are functional;

• many bones harden;

• the fetus settles in position for birth.

Fertilization occurs before pregnancy. This process takes place in one of the Fallopian tubes.

The following images summarize the stages leading to the formation of a zygote.

In the previous section, the stages (Stages 1 to 3) required for the formation of the zygote are illustrated. The embryonic stage takes place after the three stages.

The following images show how an embryo forms and makes its way into the uterus (Stages 4 to 6).

From the moment of implantation, the amniotic sac and the placenta start forming. However, they become completely functional only after 3 months.

The amniotic sac is a pouch filled with amniotic fluid, a liquid protecting the embryo by cushioning sudden movements.

The placenta is an organ containing many blood vessels that is connected to the baby by the umbilical cord. The placenta allows exchanges between the mother’s blood and the embryo’s (later fetus’) blood. It ensures gas exchanges and the supply of nutrients, while allowing the removal of waste from the embryo.

Towards the end of the 8th week of pregnancy, the organs of the baby-to-be are not yet fully developed. However, most organs are already in place. The embryo has reached a more advanced stage of development: it is now a fetus. As the weeks pass, the heart, brain, and other organs become more complex. The fingers, nose and mouth mature and become better defined.

Around the 24th week, the fetus measures about thirty centimeters and most of the organs are functional.

Childbirth occurs at the end of pregnancy. It usually happens between the 38th and the 40th week. If it happens earlier, the baby is called premature.

Towards the end of pregnancy, contractions caused by the increase in the size of the uterus and release of certain hormones trigger the onset of labour. Labour is the series of events that take place during the different stages of childbirth. The stages include dilation, birth, and delivery. The duration of childbirth varies case-by-case.