Sperm, Eggs and Embryo Development


The Sperm and Egg Journey

Millions of sperm are ejaculated during sexual intercourse. Following ejaculation the sperm enter the vagina, and thousands of them will compete and swim through the cervix into the womb and onto the fallopian tube. Many sperm will bind to the outside of the egg, but just one will penetrate the outer part of the egg resulting in normal fertilisation.

The egg starts its journey following ovulation from the ovary, and is picked up by the fallopian tube from the ovary. At the point of fertilisation the egg continues its journey through the fallopian tube and over a period of days, eventually enters the womb where it’ll try to implant and the start of a pregnancy occurs.

Embryo Development & Implantation

There are several stages that have to occur for normal embryo development and implantation to take place in the womb.

• Ovulation

The ovaries contain all of a woman’s ovarian follicles, and these follicles contain immature eggs. As a woman’s period begins her natural hormones instruct the ovaries to recruit a few of these follicles, but then over several days a dominant follicle results. Around the middle of the menstrual cycle there is a hormonal surge, and the dominant follicle releases an egg.

• Egg transport

The fallopian tube picks up the egg from the surface of the ovary, and this marks the start of the journey of the egg down the fallopian tube on route to the womb. As the egg makes its way through the fallopian tube, if a couple have sexual intercourse at the right time in their cycle, then the sperm has a chance of reaching the egg.

• Egg Fertilisation

A normally fertilised egg will only occur if a single healthy sperm enters a healthy egg. Fertilisation takes place in the fallopian tube, not the womb. Following successful fertilisation, the normally fertilised egg then begins to divide, and embryo development begins.

• Embryo Development

Following fertilisation you start to see an early cleavage stage embryo develop. A cleavage stage embryo occurs on day 2-3 after fertilisation. The fertilised egg divides and multiplies in cell number up to four to eight cells at this stage. As the embryo continues to divide further, and increases its size and cell number, the embryo continues to travel through the fallopian tube and into the womb. Around 5 days after fertilisation has taken place, the embryo is around 200 to 250 cells in number, and this is called a blastocyst.

• Implantation

As the blastocyst enters the womb it starts to interact with the womb lining (endometrium). The blastocyst continues to expand in size and the different cell types become more evident. The inner cell mass goes on to become the baby (fetus), and the prominent trophectoderm cells around the outside of the embryo go on to develop to become the placenta.