Fertility and conception

The importance of timing it right

Whether you are just thinking about trying for a baby, or are actively trying, gaining a better understanding about how your body works can be a big help. In fact one in two couples could be trying to conceive on the wrong days of a woman’s menstrual cycle[i] because they don’t have the knowledge that we are about to share with you here. We have put together information which will help you understand your fertility, the ovulation process and how this fits within your menstrual cycle as a whole.

There are only a few days (typically 6) each cycle when you can get pregnant, the days leading up to and including the day of ovulation. So knowing when these days are in your cycle is key if you are trying for a baby.

What is Ovulation?

Ovulation is the name of the process that happens once in every menstrual cycle when hormone changes trigger an ovary to release an egg. This usually happens 12 to 16 days before your next period starts.

  • The eggs are contained in your ovaries. During the first part of each menstrual cycle, one of the eggs is being grown and matured.

  • As you approach ovulation, your body produces increasing amounts of a hormone called estrogen, which causes the lining of your uterus to thicken and helps create a sperm friendly environment.

  • These high estrogen levels trigger a sudden increase in another hormone called LH (luteinising hormone). The so-called LH surge causes the release of the mature egg from the ovary - this is ovulation.

  • Ovulation normally occurs 24 to 36 hours after the LH surge, which is why the LH surge is a good predictor for peak fertility.

The egg can only be fertilised for up to 24 hours after ovulation. If it isn’t fertilised the lining of the womb is shed (the egg is lost with it) and your period begins. This marks the start of the next menstrual cycle.

While an egg only survives for up to 24 hours, sperm can remain active for up to five days. It may therefore be surprising to learn that a couple can conceive through sexual intercourse four to five days before the egg is released.

The total ‘fertility window’, taking into account the lifetime of both the sperm and the egg, is about 6 days.


What is a Fertility Window?

Your ‘fertility window’ is the period of about 6 days during your menstrual cycle when it is possible to become pregnant from unprotected sex.

The graph below shows the estimated likelihood of conception on the day of ovulation and the 5 days before.


Understanding your unique menstrual cycle

As we said above, ovulation usually happens 12-16 days BEFORE your next period starts, so the timing of ovulation depends on your menstrual cycle length. It is true that the average menstrual cycle length is 28 days, but none of us are ‘average’ and cycle lengths vary from woman to woman and cycle to cycle - in fact 46% of menstrual cycles vary by 7 or more days[ii]. So to get pregnant, it’s important to have intercourse on your fertile days and if you want to find out when you’re most fertile, it’s important to get to know your own body and your own personal menstrual cycle.

To calculate the length of your menstrual cycle, count the number of days from the first day of your menstrual bleed to the day before the next bleed starts.

The menstrual cycle length may vary from woman to woman and from cycle to cycle and typically varies between 23 and 35 days

If you’d like to have a more in-depth look at what happens inside your body during your menstrual cycle, click through to our Cycle and Ovulation page.

See your full menstrual cycle in detail

How to find your fertility window

  • [i] Johnson, SR et al. Human Reproduction (2011) 26: i236
  • [ii] Creinin MD et al. Contraception (2004) 70: 289-92

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