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How fertility works - the beginners guide

14 August 2020

When we are trying to conceive, we need to understand how our body works, which hormones are involved and identify the key days in our reproductive cycle.

Reproductive organs

First things first, what is our reproductive system? It’s a group of organs that prepare themselves for the possibility of pregnancy each month. They include the ovaries, the fallopian tubes, the uterus or womb and the vagina. These organs all play a part in our menstrual cycle, which lasts between 21 or 35 days and centres around the release of an egg from the ovaries, known as ovulation.

Menstrual cycle

Every cycle starts with menstruation or a period, the name we give to losing the lining of your womb. Day one of your period is when you see a proper red flow of menses. If this flow starts before midday, then this is day one of your period but if it comes later in the afternoon, then your day 1 will be the following day.

A period can last between two to seven days. Day one until ovulation is called the follicular phase, characterised by growing follicle which will be released in ovulation. In a 28-day cycle, this phase takes around 14 days.

The follicle grows thanks to Follicular stimulating hormone (FSH). This hormone is released by the anterior pituitary located at the base of the brain near the hypothalamus. While the follicle grows, it produces oestrogen which acts in the lining of the womb and thickens it in preparation to receive a fertilised egg. Oestrogenpeaks just before ovulation happens.


When the dominant follicle reaches a good size, around 18 mm or above, the oestrogen level in our blood is high enough to cause the pituitary gland to release Luteinizing hormone (LH), which triggers ovulation. This hormone can be detected by your ovulation test. 24 –36 hours after that surge, the mature egg leaves the ovary and is alive for 24 hours. Women are most fertile the day before and the same day of ovulation. So, it is very important to be able to identify these days: pay special attention to your body signs and your ovulation test.

What should you be looking for? Watch out for a wetter, clearer and slippery cervical mucus, tender breasts, and a bloated stomach or mild tummy pain. We may also notice an increased sexual desire and a sustained basal body temperature rise. When an egg is released, it makes its way along the fallopian tube where it might meet a sperm if you have had sex. If they do meet and fuse, this is known as fertilisation.

Ideally, the sperm is already waiting for the egg before it is released as it only lives for 24 hours.

What happens after ovulation?

After ovulation takes place, the luteal phase of the menstrual cycle starts with two possible outcomes: conceiving, or not.

If conception happens

The fertilised egg moves through the fallopian tube to the uterus. Then it implants itself in the thickened lining where it continues to grow. The Corpus luteum forms in the place in the over vacated by the egg. This physiological cyst releases progesterone, a hormone needed to keep the lining of the womb thick, which in turn helps implantation. The implanted embryo, specifically syncytiotrophoblasts cells, releases Human chorionic gonadotropin (hCG).

That hormone, which can be measured in the pregnancy test, supports the work of the ovarian corpus luteum ensuring there is enough progesterone released to maintain the endometrial lining and the pregnancy until the placenta starts working (around 10-12 weeks of pregnancy).

If the egg is not fertilised

If the egg is not fertilised or the fertilised egg does not implant itself, the corpus luteum degenerates and the level of hormones produced in the ovary (progesterone and oestrogen) begins to fall. This causes the lining of the uterus to also break down and fall away during menstruation or period, which marks the beginning of a new cycle.

Take-home messages

  • Learning about our reproductive anatomy, and the hormones and changes that can affect it, is very important to predict our fertile window and improve our chance of conceiving.

  • Identifying abnormalities in our cycle is also important so we can seek timely professional advice.

  • Knowing our fertile window, we can focus our sexual intercourse around that period. Try to have intercourse at least 3 times that week.

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