In most women one egg is released during every menstrual cycle. This usually occurs in 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 mature eggs, the number of which varies with age. 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 using an ultrasound scan, which produces a picture using harmless sound waves. On the scan, the follicle appears as a black circle in the grey background of the ovary. When the follicle reaches a certain size and the egg is considered mature, a second hormone, Luteinizing Hormone (LH), is released from the pituitary gland. This starts the mechanisms that lead, some 36 hours later, to ovulation (the releasing of the egg). The hormone‐producing cells in the follicle produce the sex hormone osetradial. This is released into the blood stream 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 the developing embryo. The egg is collected by the fimbria, the ‘fingers’ at the opening of the fallopian tube, which is called 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 pass into the egg. A reaction then takes place in the egg so that no more sperm can enter. The fertilised egg (now called an ‘embryo’) 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 6 to 7 days 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 disintegrate and is shed as a period.