There's no way to know for sure exactly when you ovulated and conceived.
As a result, most people, including health professionals, will date your pregnancy based on your LMP (last menstrual period) instead.
To work out how many weeks pregnant you are by this measure, try to remember when your last period started.
You can find out your estimated due date using our handy due date calculator.
Some women prefer to count their pregnancy from the day that they were likely to be ovulating during the month in which they became pregnant.
This will give you an estimated date of conception, which will usually be about two weeks after the first day of your LMP.
So you will be two weeks further along in your pregnancy if it's calculated according to your LMP rather than your estimated date of conception.
So, that's why Baby Centre, and most health professionals, will be telling you that you've been pregnant for about two weeks longer than you really have been.
The due date calculation works best if your menstrual cycle is regular and your periods are every 28 days.
If you have an irregular menstrual cycle, you may not know the date of your LMP.
And if your cycle varies in length, counting from the first day of your LMP may not give you a date you can rely on.
Your first ultrasound scan, called your dating scan, will give you a more accurate due date for your baby.
The person performing the scan (sonographer) will measure your baby from his head to his bottom.
This is called his crown rump length (CRL), and it gives a more accurate idea of how far along your pregnancy is You should be offered a dating scan when you're between 10 weeks and 13 weeks plus six days pregnant, based on your LMP.