The estimated day of conception is the day you first become pregnant, most people don't count pregnancy from that date. That's because there's no way to know for sure on which day you actually ovulated and conceived.
Try to remember on which date your last period started. You can work out the date your baby is likely to be born by counting from the first day of your last menstrual period (LMP). This is how most health professionals will work out your due date.
Your pregnancy should last nine calendar months and seven days, or 40 weeks. So add this length of time from the first day of your LMP to get an idea of when your baby will be born.
If you count pregnancy from your estimated date of conception, or the day that you were likely to be ovulating during the month that you became pregnant, then this will be about two weeks later.
If don't know date of last period?
The due-date calculation works best if your menstrual cycle is regular and your periods are every 28 days.
But 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.
Try not to worry. Your first ultrasound scan, called your dating scan, will give you a more accurate due date for your baby. This is calculated by measuring your baby's length from his head to his bottom. This is called his crown rump length. You should be offered a dating scan when you are between 10 weeks and 13 weeks plus six days pregnant.
When considered pregnant?
You're not actually pregnant for the first two weeks after the first day of your LMP. Although pregnancy is counted from the first day of your LMP, you're not likely to ovulate and conceive a baby until about two weeks after that day.
In fact, you probably won't find out that you're pregnant until you're about five weeks along, or have completed four weeks of pregnancy, when you've missed your next period.