The beginning of life

The egg released from the ovary meets sperm in the fallopian tube. Although many sperm gather around the egg, only 1 passes through its membrane and enters. The sperm's genetic material joins that of the egg: this is fertilisation, the start of the miracle of life. Fertilisation begins when a single sperm enters the egg and ends with the formation of a zygote. It takes about 24 hours. A sperm can survive for about 48 hours. Approximately 300 million sperm are deposited in the vagina during intercourse. Only 1%, about 3 million, enter the uterus. They then travel through the reproductive tract to the tubes. After this 10-hour journey, those reaching the egg must penetrate its covering. The egg is surrounded by a membrane called the zona pellucida. Only 1 sperm can penetrate it. This process takes about 20 minutes. Once a sperm has entered, the zona pellucida closes completely to prevent another from entering. The nuclei of the sperm and egg unite to form the zygote, completing fertilisation.

The single-celled zygote within the zona pellucida starts dividing 1.5-3 days after fertilisation. The first division is called cleavage, and the resulting cells are called blastomeres. Rapid division follows. The number of cells doubles approximately every 20 hours, and the blastomeres within the zona pellucida become smaller with each division. At 16 cells, the zygote is called a morula. Throughout this process, it continues travelling along the tube towards the uterine cavity. By day 4, the morula has reached the uterus. Cell division continues rapidly and a cavity starts to form in the centre. This structure is called a blastocyst. Although the zona pellucida remains the same size, the cells inside flatten and become more densely packed. Cell differentiation also begins: some cells become embryoblasts, which will form the embryo, while the others become trophoblasts, which will form the placenta.

Implantation into the uterus begins 5-6 days after fertilisation. The blastocyst breaks out of the zona pellucida; this is called hatching. The trophoblasts secrete an enzyme that changes the uterine lining and provides a site for implantation. At the same time, they begin secreting hCG, the hormone that makes pregnancy tests positive. After implantation, blood flow to that part of the endometrium, the uterine lining, increases, establishing the circulation needed to sustain pregnancy.

The baby's sex is determined at fertilisation. If the fertilising sperm carries an X chromosome, the baby will be a girl; if it carries a Y chromosome, the baby will be a boy. The father therefore determines the baby's sex; the mother has no role in this determination. At this stage, sex can only be identified through genetic testing. Sex is not the only feature already established: the entire genetic makeup of the individual who will be born in 38 weeks, from eye colour to blood group, has been determined and cannot change.

Changes in the mother
There are no physical or emotional changes in the mother at this stage. There may only be light spotting during implantation.