How hormones affect the menstrual cycle?
The average cycle is 28 days but, for some women, it is as short as 21 days, for others it is as long as 35 days. Every month there is a complex interaction between the pituitary gland in the brain, the ovaries and the uterus (or womb). When you first start having periods, it can also take a while before your periods develop a regular pattern.6
Day one of your cycle is the first day of your period. This is when your uterus starts shedding the lining it has built up over the last 28 days. After your period is over, the lining of your uterus starts to build up again to become a thick and spongy ‘nest’ in preparation for a possible pregnancy. On day 14 (for most women), one of your ovaries will release an egg, which will make its way through a fallopian tube and will eventually make its way to your uterus (called ovulation). On day 28 (for most women), if you have not become pregnant, the lining of your uterus starts to shed. This is your period. The blood you lose during your period is the lining of your uterus.6
How does pregnancy happen?
Pregnancy happens when a man’s sperm fertilizes a woman’s egg, which can happen even if you’ve not had sexual intercourse (penetration).7
During sex, semen is ejaculated from the man’s penis into the woman’s vagina. A man's semen (the liquid produced when he ejaculates or "comes") contains millions of sperm. One ejaculation can contain more than 300 million sperm.7
A woman's ovaries release one or more eggs (ovulation) 12-16 days before her next period. The man’s sperm enters the woman's body through her vagina, then travels through her cervix and womb to the fallopian tubes, where an egg is fertilized (conception). The egg can be fertilized by sperm contained in semen or pre-ejaculate.7