Page 6 - SONO 260 THE FIRST TRIMESTER
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                                       ž The implantation of the blastocyst within the endometrium may cause some women to experience a small amount of vaginal bleeding.
¡ This is referred to as implantation bleeding.
ž The fourth week of gestation is an extremely dynamic stage
in the pregnancy.
ž The primary yolk sac regresses during week 4 and two separate membranes are formed.
ž The outer membrane is the chorionic sac or gestational sac.
ž Within the gestational sac is the amnion or amniotic sac.
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