The conception window
The conception window is relatively brief. The egg is released (ovulation), and this is the limiting factor in conception. Although sperm can survive for up to 5 days inside a woman, they need a viable egg to fertilise to initiate pregnancy. The released egg is only fertile for up to 24 hours.
After your period, your body starts preparing your uterine lining with a thickened layer of endometrium. Part of the endometrium forms the permanent base, called the basal layer, while the other is shed as part of your period if you don’t fall pregnant, and this is called the functional layer.
The functional layer develops after your period to provide a cushion which the fertilised egg can come to rest in and start to develop.
Because sperm needs a viable egg to fertilise, under normal circumstances, conception can only occur inside this window. But because sperm can survive for up to 5 days within the cervix, sex doesn’t necessarily need to occur on the day you ovulate.
What happens after conception?
Once the egg is fertilised it will travel down into the uterus and settle in the functional layer of endometrium, where it will start dividing and developing.
This means that if you know the length of your menstruation cycle (the time between when you get your period) you can figure out your conception window because you’ll know roughly when you ovulate. This will be around the halfway mark of your cycle, usually about day 14 in a 28-day cycle. Having sex within the 5-6 days prior to this will ensure that sperm are present to fertilise the egg when it’s released.
This first 14 days prior to ovulation (and therefore conception) is actually included in your 40 weeks of pregnancy and used to calculate your due date. This means that, in the unlikely instance of having your baby on your due date (between 1 in 10 and 1 in 20 women do) you are really only pregnant for 38 weeks.