Human reproduction and gestation

In a nutshell

Reproduction is the creation of new life. Humans reproduce by sexual reproduction, during sexual reproduction sperm from a male has fertilised an egg from a female forming an embryo. This embryo develops into a foetus and the foetus grows inside the mother's uterus during gestation. After $40\ weeks$ , a baby is born.

Fertilisation and development

Fertilisation and development occurs in five stages.

1.	Ovulation	An egg is released from an ovary of a female.
2.	Copulation	Sperm from the male is released into the female's vagina during sexual intercourse.
3.	Fertilisation	The nuclei of the egg and sperm fuse , the fertilised egg is called a zygote.
4.	Cell division	$24\ hours$ after fertilisation, the fertilised egg has divided into two. After four days it has divided into $32$ cells, it is now an embryo.
5.	Implantation	One week after fertilisation, the embryo implants onto the uterus wall and the placenta begins to form.

Gestation

Definition

The time it takes the foetus to develop in the uterus is known as gestation. The foetus relies on its mother for protection against knocks, bumps, and temperature changes, oxygen for respiration, nutrients (food and water) and removal of waste substances such as carbon dioxide.

Substances such as oxygen and carbon dioxide are exchanged between the mother and foetus through the placenta. The placenta grows on the wall of the uterus and is attached to the foetus via the umbilical cord.

1. Placenta, 2. Uterus, 3. Umbilical cord, 4. Foetus, 5. Amniotic fluid, 6. Cervix, 7. Vagina

Development of the embryo

The embryo will follow certain developmental milestones. These are listed in the table below.

Time	Description
$1\ month$	The embryo is $6\ mm$ long. It has a brain, heart, eyes, ears and legs.
$9\ weeks$	The body is now $25\ mm$ long and completely formed. It is now called a foetus.
$3\ months$	The foetus is $53\ mm$ long.
$5\ months$	The foetus is now $160\ mm$ long. It kicks and has fingernails.
$7\ months$	The foetus is now $370\ mm$ long and viable. This means it has a fair chance of surviving if it was born now.
$39\ weeks$	The foetus is $520\ mm$ long and fully developed. It is ready to be born.