A 33-year-old woman, gravida 0, comes to the office for preconception counseling. Menarche was at age 12, and her menses recur every 30 days and last 5 days. She has no chronic medical conditions, takes no medications, and has no allergies. BMI is 23 kg/m2. Vital signs and physical examination are normal. If a normal pregnancy develops, which of the following processes most immediately precedes secretion of β-hCG into the maternal circulation?
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β-hCG is a hormone that supports and maintains early pregnancy development. Levels begin to rise in the maternal serum approximately 8 days following fertilization and become detectable in the urine approximately 14 days following fertilization.
Oocyte fertilization within the ampulla of the fallopian tube initiates completion of the second meiotic division and zygote formation (Choices C and E). The zygote then travels through the fallopian tube undergoing multiple mitotic divisions (cleavage), creating smaller cells (blastomeres) known collectively as a morula (a 16-32 cell mass) (Choice B). About 3-4 days following fertilization, the morula enters the uterine cavity and also forms its own central cavity, converting it into a blastocyst.
Cellular differentiation within the blastocyst forms an inner cell mass (embryoblast) and an outer layer of trophoblasts (trophectoderm). The trophectoderm contains 2 different cell populations: undifferentiated cytotrophoblasts (ie, placental stem cells) and differentiated syncytiotrophoblasts that invade the endometrium. Following invasion, the syncytiotrophoblasts begin secreting β-hCG and other hormones (eg, progesterone, human placental lactogen) into the maternal circulation.
β-hCG prevents corpus luteum degradation, thereby sustaining ovarian progesterone production to help maintain the endometrium in a secretory state that supports early pregnancy. After 10 weeks of gestation, the placental mass and syncytiotrophoblasts volume have grown large enough to become the dominant source of progesterone production.
(Choice A) Cytotrophoblasts secrete hyperglycosylated hCG, a form of hCG distinct in function from β-hCG that promotes villous invasion in early pregnancy.
Educational objective:
β-hCG secretion begins with blastocyst implantation and syncytiotrophoblast invasion. Syncytiotrophoblast cells arise from the outer layer of the blastocyst (trophectoderm) and produce β-hCG, which maintains corpus luteum progesterone production and supports the developing early pregnancy.