likely to result in pregnancy. However, pregnancy will only occur if a sperm reaches an egg and fertilises it. The fertilised egg then has to pass through the fallopian tube into the uterus and successfully implant into the wall of the uterus. This does not always happen. This means that sexual intercourse during the fertile period does not always result in a successful pregnancy even if both the man and the woman are fertile. In some situations a couple is not able to conceive a baby naturally. The couple may then be said to be infertile. Sometimes medical treatment can help the couple to conceive.
13.7 Fertilisation
The sperm released during sexual intercourse swim up through the uterus and into the fallopian tubes. If an egg released from an ovary is present in one of the fallopian tubes, then one of the sperm cells may fuse with it. The nuclei of the egg and sperm cells fuse to form a fertilised egg known as a zygote. The fertilised egg then divides a number of times to form a tiny group of cells which moves down the fallopian tube towards the uterus.
The fertile
period The time during the menstrual cycle when sexual intercourse may result in pregnancy.
Fertility The ability to produce offspring.
Infertility The inability to
produce offspring.
Fig. 6 Microscopic view of a sperm attempting to enter an egg.
13.8 Implantation
After reaching the uterus the group of cells settles into the prepared uterus lining in a process known as implantation. After implantation the group of cells will continue to divide to form the embryo and the placenta. The process of implantation stops the uterus lining from breaking down and causes the menstrual cycle to stop.
