Fill in the Blanks

Visualization of the embryo on ultrasound is often referred to as the " fetal " and should be seen with MSD mm .
? At beginning of fifth week , bilaminar embryonic disk converts into ( three germ layer ) embryonic disk .
? Organogenesis begins at this point . ( Organs start developing )
? is the first organ to function within embryo
o Starts beating at approximately days ( 5 to 5½ weeks )
o Embryonic heartbeat must seen in living embryo when CRL mm . with no heartbeat is considered nonviable .

3 primitive germ cell layers that compromise embryonic disk by - week past LMP
o : inner layer - becomes GI & respiratory systems
o : middle layer ? becomes musculoskeletal & circulatory systems
o : outer layer ? becomes brain & nervous system , and ski n

At beginning of week of gestation , embryonic disk folds into Cshaped embryo .
o While embryonic folding continues , this results in a or narrowing between embryo and yolk sac , creating the yolk stalk
? The yolk stalk consists of ducts :
? / duct : part of the connecting stalk of the embryo that gives rise to part of the umbilical cord and bladder . Obliterates to form the urachus .
? duct / duct : During embryonic folding , the dorsal aspect of yolk sac ( vitelline duct ) is incorporated into the embryo , developing the foregut , midgut , hindgut , and forming the entire tract , liver , biliary tract , and pancreas
o The yolk stalk and are brought together by expanding amnion , which covers the three structures forming the cord .

From 6 to 10 Weeks :
? Spine develops from
o May be visualized sonographically as echogenic lines at 6 weeks of gestation .
? Cranium undergoes dramatic changes from sixth to 10th week of gestation
o By weeks , 3 vesicles seen within fetal brain :
?
? Mesencephalon
? - Divides into two segments :
? Cephalic portion : which forms cerebellum and pons
? Caudal component : which forms the medulla
o Once rhombencephalon divides , the cystic rhomboid fossa forms : a structure , seen within posterior aspect of embryonic cranium
? This should not be confused with pathology , such as - Walker malformation .
? rhomboid fossa can routinely be imaged sonographically from
eighth to 10th week of gestation .

By 9 weeks the midline is formed and the echogenic choroid plexus tissue seen this produces . The lateral ventricles completely fill cerebral vault
? At weeks further evolution of cerebellum , , and medulla oblongata encloses rhomboid to form primitive fourth ventricle and part of cerebral aqueduct of Sylvius .
? Cerebellum is fused and brain structure complete shortly thereafter

More 6 - 10 weeks changes :
o Limb buds and embryonic are recognizable during sixth week of gestation
o Sonographically , limb buds may be detected from week on .
o Hands and feet develop later in first trimester and completely formed by end of week of gestation
o Fingers and toes have been identified using sonography at 10 weeks
o Limbs not routinely identified until of long bones begins at 10 weeks
o 8 weeks - Calcification of begins , followed by ossification of mandible , palate , vertebral column , arches
o 9 weeks - Frontal bones begin to calcify followed by long bones
o 10 weeks - Further palate development may be visualized

Anterior abdominal wall is developed by weeks gestation
o The gut is formed simultaneously as the dorsal yolk sac is incorporated into the embryo . It then develops into the midgut .
o forms the majority of small bowel , cecum , ascending colon , proximal transverse colon
o As amniotic expansion occurs , midgut faster than embryo , causing midgut to herniate into the base of umbilical cord
o So because there ? s not enough room in the abdomen for the midgut ( bowel ) to grow at it ? s normal rate , it grows and develops to the fetal abdomen and then migrates back in when there ? s enough space .
? This transition of bowel can readily be visualized as an mass within base of umbilical cord .
o Until weeks of gestation , midgut loop continues to grow and rotate before it descends into fetal abdomen at about the 11th week .
o After weeks of gestation , this echogenic umbilical cord mass is no longer visualized and echogenic bowel is now seen within fetal abdomen
o It is important that this normal embryologic event not be confused with pathologic processes , such as or

? By end of week of gestation , heart has obtained its adult configuration .
o Embryonic cardiac rates vary with gestational age . to 115 bpm at 6 weeks , to 160 bpm at 9 weeks .
o Documenting fetal heartbeat of over bpm provides discrimination from maternal heartbeat .
? If unsure of maternal vs fetal heart rate , or is fetal heart rate is low , obtain and document puls e

? Transvaginally , the amniotic membrane that separates the amniotic cavity and chorionic cavity is routinely seen after weeks
? " double sign " - simultaneous side by side appearance of amnion and yolk sac
o Gradually , the takes over process of waste removal , amniotic cavity expands , and chorionic cavity decreases in size .
? Fusion of membranes ( fusion ) occurs at approximately 14 to 15 weeks and should be complete by week s