Merck Manual

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Video Sonogram of 30-Day Conceptus
Video Sonogram of 30-Day Conceptus
Video Sonogram of 30-Day Conceptus

At 30 days of gestation the dorsal yolk sac is equal in size to the developing allantoic cavity. A heartbeat should be evident in the embryo proper after 25 days of gestation.

Courtesy of Dr. Patricia Sertich.