One thing that’s struck me over the years is how little we generally know about our menstrual cycle.  Until we start trying for a baby we’re usually only aware of it in relation to getting our period every month. Some of us also get a bit of a niggle around ovulation time, but that’s pretty much it.

Talking to someone one day about her friend who was struggling to conceive, I was struck by how we sometimes overlook the obvious when we think about reasons why people struggle to conceive.  She said to me “I’m sure she’s just having sex at the wrong time every month”. Until that point it hadn’t really been a possibility I’d considered.  I suppose I’ve been specialising in this area for so many years that I take it for granted that we all know how our reproductive systems work – but why would we necessarily?

So when you’re trying to conceive, it’s important to have a basic understanding of the way the female reproductive system works.

A summary of the female reproductive cycle

There are 4 phases in the female reproductive cycle:

1. Menstruation (period)

In the second week of your cycle, your body prepares the uterus for a fertilised egg to implant. If this doesn’t happen, this thickened lining of the uterus (endometrium) is eliminated from the body through the vagina. Menstrual fluid contains blood, cells from the lining of the uterus (endometrial cells) and mucus. The average length of a period is around 5 days.

2. Follicular phase

The follicular phase starts on the first day of your period and ends with ovulation. Prompted by the hypothalamus gland in the brain, the pituitary gland releases follicle stimulating hormone (FSH). This hormone stimulates the ovary to produce around five to twenty follicles (tiny nodules or cysts) on its surface.

Each follicle contains an immature egg. Normally only one follicle will mature into an egg, while the others die. This can occur around day 10 of a 28-day cycle. The hormones produced by the growth of the follicles, stimulate the lining of the uterus to thicken in preparation for possible pregnancy.

3. Ovulation

Ovulation is the release of a mature egg from the surface of the ovary. This generally occurs mid-cycle, around 14/15 days before menstruation starts. During the follicular phase, the developing follicle causes a rise in the level of oestrogen. The hypothalamus recognises these rising levels and releases a hormone called gonadotrophin-releasing hormone (GnRH). This prompts the pituitary gland to produce raised levels of luteinising hormone (LH) and FSH.

Within two days, ovulation is triggered by these high levels of LH. The egg is pushed through into the fallopian tube and towards the uterus by small, hair-like projections. The life span of the typical egg is around 24 hours. Unless it meets a sperm during this time, it will die.

4. Luteal phase

During ovulation, the egg emerges from its follicle, but the ruptured follicle stays on the surface of the ovary and for the next two weeks or so, it transforms into a structure known as the corpus luteum. This structure starts releasing progesterone, along with small amounts of oestrogen. This combination of hormones maintains the thickened lining of the uterus, waiting for a fertilised egg to implant.

If a fertilised egg implants in the lining of the uterus, it produces the hormones that are necessary to maintain the corpus luteum. This includes human chorionic gonadotrophin (HCG), the hormone that is detected in a urine test for pregnancy. The corpus luteum keeps producing the raised levels of progesterone that are needed to maintain the thickened lining of the uterus.

If pregnancy does not occur, the corpus luteum withers and dies; usually around day 22 in a 28-day cycle. The drop in progesterone levels causes the lining of the uterus to fall away and be released as a period. The cycle then repeats.

So when’s a good time to try for a baby?

Knowing when you ovulate is important when you want to get pregnant, because the window of opportunity to conceive is fairly small every month. Conception is only possible from around five days before ovulation through to ovulation itself. These six days are the “fertile window” in a woman’s cycle and reflect the lifespan of sperm (five days) and the lifespan of the egg (24 hours). The likelihood of conceiving is significantly increased if sex occurs in the three days leading up to and including ovulation. Where there are no gynaecological issues, the probability of pregnancy rises steadily during this time and is 27-33% in the three days leading up to and including ovulation. From that point, the probability of pregnancy declines rapidly. Twelve to 24 hours after ovulation, a woman is no longer able to get pregnant during that cycle.

Your body is pretty clever and does give you some signals around ovulation. There are a few tips in this article which you might want to check out:

If all this seems too complicated, or if you have an irregular cycle and ovulation day is hard to predict, an alternative is to have sex every two to three days. That way all bases are covered without getting too concerned about when the chance of conceiving is greatest.


If you have been trying to conceive for a year or more (6 months if you’re 40 or over), please go and see your GP.  There is a system of medical testing and intervention in place to help you.  This blog post is intended as guidance only, and should not be considered a substitute for medical advice.

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