A repeat ultrasound will provide valuable information about the growth of the fetus and may reassure you and your doctor that the change in due date is reasonable. Ultrasound measurements for estimating the age of a fetus are more accurate during the early stages of pregnancy.
In the first few weeks, fetuses tend to develop at the same rate.
In these cases, a pregnancy wheel can still be used, but some simple calculations will be necessary.
During the first trimester, this measurement provides the most accurate estimation for the age of the baby.
Later ultrasounds are helpful in determining whether the fetus is growing well, but not in determining the age of the fetus.
OBJECTIVE: To analyse the association between fetal size at time of dating ultrasound and risk for preterm delivery and small-for-gestational-age (SGA) birth and to evaluate if timing of ultrasound, that is before 14 weeks of gestation or after 16 weeks affects this association. SETTING: Ultrasound departments of Ultragyn, Stockholm, Sweden.
This is the time from ovulation to the next menstrual period.
If your cycle is 35 days long, for example, then you probably ovulated on day 21.