VMBO Year 3 · Reproduction and sexuality · Section 2.4
Fertilisation and pregnancy
What happens during fertilisation?
Fertilisation usually occurs in an oviduct. One sperm enters the egg, triggering changes that prevent other sperm from entering. The genetic material from the sperm and egg combines, forming a zygote. This first cell divides repeatedly while the whole group of cells initially remains about the same size. It travels towards the uterus.
Implantation and pregnancy
About a week after fertilisation, the early embryo begins implantation in the uterine lining. Cells which will contribute to the placenta produce hCG, a hormone maintaining the corpus luteum. This helps keep the uterine lining in place, preventing menstruation. A pregnancy test detects hCG, usually reliably from around the time a period is due.
Development of the embryo
The embryo grows through cell division and development. The placenta forms where maternal and embryonic tissues meet. It allows oxygen and nutrients to pass from the mother to the embryo, and carbon dioxide and other wastes to pass in the opposite direction. The two blood supplies normally remain separate.
The embryo is connected to the placenta by the umbilical cord. It usually contains two arteries and one vein. The vein carries oxygen-rich blood towards the embryo; the arteries carry blood back towards the placenta.
Protection of the embryo
Foetal membranes surround a fluid-filled space containing the developing baby. Amniotic fluid cushions shocks, prevents drying out and helps keep the temperature stable. It also allows movement. From about the ninth week after fertilisation, the developing baby is called a foetus. By then, the basic structures of the main organs have formed and continue to develop. Pregnancy weeks used in healthcare are usually counted from the last menstrual period, about two weeks before fertilisation.
Identical and non-identical twins
Non-identical twins develop when two eggs are fertilised by two sperm. Each forms a separate embryo. Identical twins develop from one fertilised egg when the early embryo splits into two groups of cells, each developing into an embryo.
Glossary
- Fertilisation
- The combination of genetic material from an egg and a sperm, producing a zygote.
- Identical twins
- Twins developing from one egg fertilised by one sperm, followed by splitting of the early embryo.
- Embryo
- The developing organism during the early stages after fertilisation, including the period of implantation and organ formation.
- Foetus
- The developing baby from about nine weeks after fertilisation until birth.
- Implantation
- The attachment and embedding of the early embryo in the uterine lining.
- Afterbirth organ
- The placenta, formed at the interface between maternal and embryonic tissues, exchanging nutrients, gases and wastes.
- Umbilical cord
- The connection between the developing baby and placenta, usually containing two arteries and one vein.
- Placenta
- The organ exchanging oxygen, nutrients and waste products between the mother and developing baby without normally mixing their blood.
- Non-identical twins
- Twins developing from two eggs fertilised by two different sperm.
- Foetal membranes
- Membranes surrounding the developing baby and amniotic fluid.
- Amniotic fluid
- Fluid cushioning the developing baby and protecting against drying out and temperature changes.