Understanding fertility

Fertility explained

Fertility is the ability to conceive a child. Fertility may not be at the forefront of your mind during the majority of your reproductive life and may only be thought of once a decision has been made to start a family. The process of reproduction is complex, and it is useful to understand how eggs and sperm are normally formed, and how conception occurs to fully understand the process of reproduction.

Fertility requires healthy sperm to successfully make its way to a healthy egg, penetrate the egg and then the embryo (egg fertilised with sperm) successfully attaches to the lining of the uterus (implantation). A problem at any of these steps can affect your fertility.

Understanding reproduction

The hormones which control the production of sperm and eggs are called gonadotropins. There are two types of gonadotropins: follicle-stimulating hormone (FSH) and luteinising hormone (LH). 

In men and people with testes, gonadotropins stimulate the testicles to produce sperm and the sex hormone testosterone. Read more information on male hormones here. Sperm are produced at the rate of about 300 million per day. They take around 80 days to develop and mature. Each sperm has a head, which contains the genetic material, and a tail, which propels it up through the vagina, uterus, and fallopian tubes where the egg is fertilised.

HPG axis of a person with testes

In women and people with ovaries, gonadotropins act on the ovaries, stimulating the growth of multiple follicles (fluid-filled sacs) where the eggs develop. The female sex hormone, oestrogen, is produced by the ovaries as the eggs mature within the follicles. The rising levels of oestrogen help to release an egg (ovulation) and after ovulation, another female sex hormone, progesterone, helps prepare the lining of the uterus for the implantation of an embryo.

The production of sex hormones and the release of an egg is known as the menstrual cycle. It is counted from the first day of the period until the day before the start of the next period and can be broken down into 4 phases:

•    menstruation
•    the follicular phase
•    ovulation
•    the luteal phase

Read more information about the menstrual cycle here and here.

In an average cycle of 28 days, ovulation occurs around day 14. However, cycle length varies between women and people that menstruate, and it is important to note that ovulation occurs earlier in those with shorter cycles and later in those with longer cycles.

HPO axis of a person who mesntruates

Conception occurs when an egg and a sperm come together. At the time of ovulation, an egg is released from the ovary into the fallopian tube and can only live for 12-24 hours. If sperm are present at that time of ovulation, the egg can be fertilised. The fertilised egg then starts to develop into an embryo and travels along the fallopian tube to the uterus. After ovulation, the lining of the uterus - the endometrium – prepares for the impending implantation of the growing embryo. A few days after implantation, the embryo starts to produce human chorionic gonadotrophins (hCG) - the hormone that gives a positive pregnancy test reading.

If an embryo does not form or attach to the endometrium, the level of progesterone hormone drops, the lining of the uterus breaks down causing the next period to begin. 

Factors affecting fertility

Fertility can be affected by many things, including how old you are, when you have sex, how healthy you are, and whether you have any medical conditions. Planning ahead and taking care of your preconception health by modifying your lifestyle can improve your chance of pregnancy and the health of your future child. Read more about the modifiable factors that can affect your fertility here. Medical conditions such as polycystic ovary syndrome (PCOS) and endometriosis can also reduce fertility, however, it may just take longer to get pregnant. People may find themselves trying to get pregnant at an older age when it can be a lot more difficult to conceive. Around 1 in 6 people may experience difficulties when trying for a baby. If you are having trouble conceiving, read more information here.

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Frequently Asked Questions

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I want to get pregnant. When is the best time to have sex?

Your Fertility’s ovulation calculator can help you work out the fertile window. These are the days a woman is most likely to get pregnant.

Where can I find more information about men’s health and fertility?

  • The Healthy Male website provides the latest scientific and medical research on male reproductive and sexual health.
  • The Your Fertility website provides facts about fertility, so that you can make the best possible decisions about having children.
  • Make an appointment to talk to your GP about preconception health and trying for a baby.

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