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Diagnostic ultrasound ( PDFDrive )

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1058 PART IV Obstetric and Fetal Sonography

A

B

C

D

FIG. 30.9 Normal Yolk Sac. (A) Sagittal and (B) transverse TVS of the early IUP demonstrates the yolk sac at 5 weeks 5 days. (C) Yolk sac

is seen separate from embryo (calipers) at 9 weeks. (D) At 11 weeks, yolk sac lies at edge of chorionic sac outside of early amnion.

ys

e

FIG. 30.10 Normal Embryo at 8 Weeks. TVS shows vitelline duct

(thick arrow), yolk sac (ys), and embryo (e).

FIG. 30.11 Early Monochorionic Diamniotic (MCDA) Twins. Two

separate yolk sacs are seen within a single chorion at 5 weeks 5 days.

It is too early to visualize the two amnions.

Embryonic Cardiac Activity

Using TVS, an embryo with a CRL as small as 1 to 2 mm may

be identiied immediately adjacent to the yolk sac (Fig. 30.15).

In normal pregnancies the embryo can be identiied in gestational

sacs as small as 10 mm and should always be identiied when

the MSD is equal to or greater than 25 mm with optimal scanning

parameters and high-resolution TVS. 4,5

Embryologic data suggest the tubular heart begins to beat at

36 to 37 days’ gestational age. 10 Cadkin and McAlpin 47 described

cardiac activity adjacent to the yolk sac before the embryo can

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