The best time to date a pregnancy with ultrasound is between the 8th and 18th weeks of pregnancy. Women who have undergone special procedures to become pregnant, such as artificial insemination or in vitro fertilization, usually know exactly when conception occurred. In these cases there is no doubt about the baby's gestational age. You cannot remember the date of your last period. The baby is unusually large or small.
Some doctors use early ultrasound to estimate gestational age and due dates for all pregnant women.
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