Use our Pregnancy Calculator to calculate these dates for you.If the mother has a regular period and knows the first day of her last menstrual period, gestational age can be calculated from this date.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.
The EDD from the early dating scan is used - if the last menstrual period is not known or is unreliable, or the dating scan differs from the last menstrual period dating by more than 5 days.
For example if you a tiny sac can be seen in the uterus, but the baby and its heart beat may not be detected yet.
5 ½ weeks gestation means 5 ½ weeks from the first day of the last menstrual period, which is usually about 3 ½ weeks from the date of conception (confusing, isn’t it!
In some cases, it is difficult to determine the gestational age because the baby is unusually large or small.
Also, in some cases the size of the uterus in early pregnancy or the height of the uterus in later pregnancy does not match the first day of the last menstrual period.