Ans: a (b hCG) Ref: Ganong, 22nded, p. 449In humans, after fertilization corpus luteum enlarges in size (instead of regression as occurs in normal enstrual cycles). This occurs in response to human chorionic gonadotropin secreted by placenta.Corpus luteum:LH is responsible for the initial formation of corpus luteum. In the absence of pregnancy it begins to degenerate by 24th day ( 8th post ovulatory day) of the cycle and eventually gets replaced by corpus albicians. But during pregnancy corpus luteum is maintained till the 10th week of gestation by b hCG and it is the major source of progesterone during this period.hCG:* It's a glycoprotein with alpha and beta subunits.* Alpha subunit is identical to alpha subunit of LH. FSH & TSH.* Beta hCG is secreted by syncytiotrophoblasts of placenta in to both maternal and fetal blood. It is used for pregnancy test.Detected in blood as early as 6th day of conception by RIA. Serum cone, doubles every 2 days.Detected in urine by 10th day--peaks in 70 days and returns to normal by 100th day.Disappears in urine after 2 days and from blood after 2 wks of delivery.Functions of hCG:1) Stimulates progesterone production ( LH like action).2) Stimulates Leydig cells of male fetus & development of male genetalia.3) Has some TSH like activity.HCG value should double every 2 days in normal pregnancy. When values are lower than normal/ slow rise/ plateauing of hCG titre, it indicates a threatened abortion/ ectopic pregnancy.High level of hCG are seen in:Multiple pregnancyChoriocarcinomaH. moleTrisomy 21 fetus (Down syndrome)