Multiple Gestation Figure

■ Division of embryos before differentiation of trophoblast (between days 2 and 3) ^ 2 chorions, 2 amnions

■ Division of embryos after trophoblast differentiation and before amnion formation (between days 3 and 8) ^ 1 placenta, 1 chorion, 2 amnions

■ Division of embryos after amnion formation (between days 8 and 13) ^ 1 placenta, 1 chorion, 1 amnion

PREGNANCY PROTEINS

hCG (Human Chorionic Gonadotropin)

Source: Placenta Function:

■ Maintains the corpus luteum

■ Stimulates adrenal and placental steroidogenesis

Dizygotic Twins Nonidentical or Fraternal Twins

(Always have 2 chorions and 2 amnions, and sexes may be different.)

Chorion Amnion

Dichorionic Diamnionic (1/3)

Separate development

Dichorionic Diamnionic (1/3)

Fused development

Monozygotic Twins Identical or Maternal Twins

Monochorionic Monoamnionic (rare)

(Siamese twins) Twins share single cavity

Monochorionic Diamnionic (< 1/3)

Twins are in separate cavities

FIGURE 4-1. Multiple gestation.

(Reproduced, with permission, from Lindarkis NM, Lott S. Digging Up the Bones: Obstetrics and Gynecology. New York: McGraw-Hill, 1998:56.)

ACTH (Adrenocorticotropic Hormone)

Source: Trophoblasts

Function: Stimulates an increase in circulating maternal free cortisol hPL (Human Placental Lactogen)

Source: Trophoblasts

Function: Antagonizes insulin ^ maternal glucose intolerance, lipolysis, and proteolysis

CRH (Corticotropin-Releasing Hormone)

Source: Placental tissue and decidua

Function: Stimulates placental ACTH release and participates in the surge of fetal glucocorticoids associated with late T3 fetal maturation

Prolactin

Source: Decidualized endometrium

Function: Regulates fluid and electrolyte flux through the fetal membranes Alpha-Fetoprotein (AFP)

Source: Yolk sac, fetal gastrointestinal tract, and fetal liver Function: Regulates fetal intravascular volume (osmoregulator)

■ MSAFP peaks between 10 and 13 weeks' gestational age, then declines thereafter.

■ Detectable as early as 7 weeks' gestation

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