Gestational age is calculated from the first day of the mother’s last menstrual period and not from the date of conception.
The baby can be measured as early as 5 or 6 weeks after the mother’s last menstrual period.
Measuring the baby using ultrasound is most accurate in early pregnancy. The best time to estimate gestational age using ultrasound is between the 8th and 18th weeks of pregnancy.
The most accurate way to determine gestational age is using the first day of the woman’s last menstrual period and confirming this gestational age with the measurement from an ultrasound exam.
So, you don't have a last period date to go on, or you're cycles are irregular enough to not know which end is up. But for those opposed to ultrasound (or for those just wanting that first pregnancy challenge) there is another way.
For so many women it is, and many doctors insist on routine ultrasound when there isn't a period date to go on.
Very few women actually deliver on their due date, so while it’s important to have an idea of when your baby will be born, try not to get too attached to the exact date.Every pregnancy is unique and your baby will come when it's ready.Be sure to talk to your health care provider about your due date.On the flip side, if your due date is put too early and you are pressured to induce for whatever reason, you could wind with a preterm baby, which is no fun either.The bottom line is this- it is worth it to you to put together all the clues you can about your own pregnancy and conception.