One of the first things that a woman who becomes pregnant want to know (assuming she hasn’t been trying to get pregnant) is when her baby was conceived. The age of your baby is known as "gestational age," and it is calculated from the first day of your last menstrual period.
If you have a regular period of 28 days (or another number of days that you can look back at) and you know the first day of your last menstrual period, you can calculate your baby’s gestational age from that date. Of course, if you have irregular menstrual periods or if you aren’t sure when the first day of your last menstrual period was, it can be a bit harder to calculate your baby’s gestational age.
In that kind of situation, you’ll usually need an ultrasound. An ultrasound can measure your baby once you’re as little as five or six weeks after your last menstrual period. Using an ultrasound to determine your baby’s gestational age, of course, isn’t as exact as if you know when your last period began, but it can bet pretty close.
The best time to calculate gestational age using an ultrasound is between the 8th week of pregnancy and the 18th week. Before and after, measurements may not be able to accurately reflect the gestational age because the baby may be a little large or a little small.
It’s worth noting that your baby’s gestational age doesn’t have anything to do with the date of conception. This is because conception is sometimes harder to pin down than the start of your last period. Conception will usually occur between 11 and 21 days after the first day of your last menstrual period.
The obvious exception to this, of course, is when you’ve been trying to conceive. You will often know the exact date, because you timed things that way. This is also the case with women who undergo fertility procedures such as IVF.