Once the egg is fertilised it will travel towards the womb to nestle itself into the wall of uterine lining, also called the endometrium. The build up of endometrium signals your body to cease ovulating, and you will stop having periods.
The fertilised egg, now called a zygote, will have started to split from one cell, into two, then into four, and so on. Around the time that these cells reach the womb, they will have divided into over 100, at which stage the zygote can be called an embryo.
Your body is also starting to release hormones to tell it to behave differently now that you’re pregnant. If you or your partner are particularly observant, you might notice that your body temperature is higher than normal or that you are needing to urinate more often.
The cells that will form the placenta are also starting to develop, and these produce the particular hormone that can be detected in pregnancy tests. If your pregnancy is planned, you can time these self-tests to get a very early indication that you are pregnant.