In most women 1 egg is released during every menstrual cycle. This usually occurs around the middle of the cycle. The first part of the cycle, from the start of the period to ovulation, is called the follicular phase. During this phase the egg that will be released that month, is selected from a batch of approximately 20 immature eggs. Each egg is surrounded by a layer of hormone-producing cells and together they constitute what is called a follicle. The follicle that is selected grows under the influence of a hormone called follicle stimulating hormone (FSH). This hormone is released by a small gland at the base of the brain called the pituitary gland.
As the follicle grows, a lake of hormone-rich fluid forms around the egg. This can be seen by using an ultrasound scan. Ultrasound produces a picture by using harmless sound waves. On the scan, the follicle appears as a black circle in the grey background of the ovary.
Ultrasound picture of a developing ovarian follicle
When the follicle reaches a certain size and the egg is mature, a second hormone, luteinising hormone (LH), is released from the pituitary gland. This triggers the mechanisms that ultimately, some 40 hours later, lead to ovulation - the release of the egg.
The hormone-producing cells in the follicle produce the sex hormone oestradiol. This is released into the bloodstream and stimulates the lining of the uterus, known as the endometrium, to thicken. After ovulation a second hormone, progesterone, is released from the same hormone-producing cells in the ovary. Together, the oestradiol and progesterone prepare the lining of the uterus for implantation of the developing embryo.
Diagram illustrating ovarian, hormonal and endometrial changes during a menstrual cycle
The egg is collected by the fimbriae, the 'fingers' on the end of the fallopian tube and moves into the wider part of the tube known as the ampulla. If sexual intercourse has occurred, sperm will swim up through the cervix, through the uterus and along the fallopian tubes to the ampulla. Although many sperm will surround the egg only one will enter through its protective coat, the zona pellucida, and penetrate the egg. A reaction then takes place in the egg so that no more sperm can enter. The fertilised egg remains in the ampullary part of the fallopian tube for up to 48 to 72 hours before starting the journey to the uterus, arriving in the uterus in about 5 days.
The small embryo has now formed into a cluster of cells known as a blastocyst. This blastocyst comes to rest against the side of the uterus and starts to implant about day 6 to 7 after fertilisation. As implantation is taking place this small early embryo sends a signal to the ovary, which continues to secrete the sex hormones, progesterone and oestradiol. These hormones keep the endometrium favourable for the early pregnancy to continue.
If the egg fails to fertilise, the ovary will stop producing the sex hormones and the endometrium will break down and is shed as a period. The whole process then starts up again as the start of a new cycle.