How accurate are dating scans in pregnancy
Measurements taken at these early scans will help to; confirm a pregnancy, date the pregnancy and to see any problems that may occur in the pregnancy ( low lying placenta, twins etc.).Scans in early pregnancy are usually performed in the first trimester around 12 weeks.
The presence of a yolk sac confirms the diagnosis of an intrauterine pregnancy and excludes ectopic pregnancy, except in rare cases of simultaneous intrauterine and extrauterine gestations.It initially appears as a round, anechoic structure.In experienced hands, it may be detected as early as 30 days gestation by TVU.Gestational Sac week 4 ultrasound scan In these early first trimester scans, when no structures are visible within the gestational sac, gestational age (GA) may be estimated from the sac diameter.A common method is to measure the mean sac diameter (MSD), add 30 to the sac size in millimeters, to give gestational age in days.
By the time the embryo becomes visible on ultrasound the sac diameter is no longer accurate in estimating gestational age.