VMBO Year 2 · HAVO/VWO Years 2–3 · Reproduction and sexuality · Section 4.4

Becoming pregnant

Erection and ejaculation

An erection occurs when erectile tissue in the penis fills with blood, making it larger and firmer. It may occur with sexual arousal or spontaneously.

The clitoris also contains erectile tissue. During arousal, vaginal lubrication may increase. Ejaculation releases semen through the urethra. Semen contains millions of sperm cells. Ejaculation can also happen during sleep, sometimes called a wet dream.

If semen enters the vagina during penis-in-vagina intercourse, sperm may travel towards an egg and pregnancy may become possible.

Fertilisation

Fertilisation usually occurs in an oviduct when a sperm-cell nucleus fuses with an egg-cell nucleus.

Sperm move using a flagellum, or tail, and may survive for several days in suitable conditions in the reproductive tract, sometimes up to about five days. An egg can usually be fertilised for around 12–24 hours after ovulation.

After one sperm fuses with the egg, changes help prevent other sperm from entering.

Implantation

The fertilised egg divides repeatedly while travelling towards the uterus. A developing ball of cells then attaches to and enters the uterine lining. This is implantation, usually beginning roughly six to seven days after fertilisation, with timing varying.

Hormonal changes maintain the uterine lining and menstruation usually stops. Birth is typically around 38 weeks after fertilisation, or about 40 weeks when pregnancy is dated from the last menstrual period.

Pregnancy

The developing organism is called an embryo during the first eight weeks after fertilisation, and a fetus from the ninth week after fertilisation. Clinical pregnancy weeks are usually counted from about two weeks earlier.

Early development uses available nutrients and uterine secretions. The placenta, formed from fetal and maternal tissues, later supports exchange of substances between mother and fetus. Their blood normally remains in separate circulations.

The umbilical cord connects the fetus to the placenta. The fetus is surrounded by amniotic fluid and protective membranes, cushioning movement and supporting a stable environment.

Ultrasound and prenatal screening

Prenatal screening examines aspects of development during pregnancy. An ultrasound scan uses sound waves to create images; an early dating scan estimates the stage of pregnancy.

In the Dutch screening programme, NIPT can be offered from ten weeks of pregnancy. This blood test screens for selected chromosome conditions; it does not diagnose every genetic condition or provide certainty.

Scans around 13 and 20 weeks examine physical development, including organs such as the heart and brain. These tests are optional. Other scans may produce 3D or moving 4D images, but these do not replace medical screening.