OB Dating Atlas - Week 4 - Mean Sac Diameter (MSD)
Caption
A 28-year-old G0 with an unknown last menstrual period (LMP) presents for a missed period and a positive urine pregnancy (UPT) test two days ago. On abdominal ultrasound, the mean sac diameter is .92 cm consistent with a 4w6d intrauterine pregnancy (IUP). General Rules: MSD is the earliest measurement that can assess GA. MSD can be used until the embryo is present (5w0d-8w0d), then use the CRL. Mean of 3 orthogonal measurements (length, width, and height) of fluid-filled space in sac. Gestational Age (days) = MSD (mm) + 25 This equation can be used when using a portable or handheld device that does not calculate the gestational age automatically. Accuracy3: ± 5-7 days Pearls: A gestational sac alone cannot confirm a pregnancy. In the presence of an ectopic pregnancy, a pseudogestational sac may be present. An MSD > 12.0 cm with no yolk sac visualized should raise concerns for a missed abortion or blighted ovum and should be further investigated.