Ultrasound examination in the first trimester allows accurate assessment of gestational age, and identifies and allows for appropriate care of women with multiple pregnancies.
Methods used to assess gestational age include known date of ovulation, date of the last menstrual period (LMP) and diagnostic ultrasound.
Your doctor may change your due date if your fetus is significantly smaller or larger than the average fetus at your particular stage of pregnancy.
Generally, your doctor will order an ultrasound to determine the age of your baby when there’s a history of irregular periods, when the date of your LMP is uncertain, or when conception occurred despite oral contraceptive use.
If you have regular 28-day menstrual cycles, there are two ways to calculate your due date.