How accurate is a dating scan at 8 weeks
For example, if another scan done 6 or 8 weeks later says that one should have a new due date which is further away, one should not normally change the date but should rather interpret the finding as that the baby is not growing at the expected rate.
b) The Biparietal diameter (BPD) The diameter between the 2 sides of the head. It increases from about 2.4 cm at 13 weeks to about 9.5 cm at term.
The timing of a positive pregnancy test may also be helpful in this regard to assess the possible dates of conception.
Dating with the CRL can be within 3-4 days of the last menstrual period.
(Table) An important point to note is that when the due date has been set by an accurately measured CRL, it should not be changed by a subsequent scan.
Fetal body measurements reflect the gestational age of the fetus. In patients with uncertain last menstrual periods, such measurements must be made as early as possible in pregnancy to arrive at a correct dating for the patient. In the latter part of pregnancy measuring body parameters will allow assessment of the size and growth of the fetus and will greatly assist in the diagnosis and management of intrauterine growth retardation (IUGR).
a) The Crown-rump length (CRL) This measurement can be made between 7 to 13 weeks and gives very accurate estimation of the gestational age.
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Fetal heart rate tends to vary with gestational age in the very early parts of pregnancy.