A 41-year-old woman, gravida 0, comes to the office for evaluation of pregnancy. Menarche was at age 12, and her menses recur every 30 days and last 5 days. The patient and her husband have been trying to conceive for the past year and plan intercourse during her fertile window according to the ovulation predictor kit. She has no medical problems, takes no medications, and has no allergies. BMI is 23 kg/m². Vital signs and physical examination are normal. If fertilization and implantation occurred this cycle, when would the β-hCG level first be detectable in the serum?

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11ec160b_374d_f5eb_9249_e789e0d67341_MD0001_00 An ovulation predictor kit measures urinary LH and becomes positive 24 hours before ovulation. Once an oocyte is released from the ovary, sperm may fertilize it for up to 24 hours. After fertilization, the second meiotic division completes and a zygote forms. The zygote travels through the fallopian tube and undergoes mitotic divisions (cleavage), creating smaller cells (blastomeres) that are collectively known as a morula. The morula enters the uterus 3-4 days after fertilization and develops a blastocystic cavity, becoming a blastocyst. The blastocyst implants 6 days after fertilization (Choices A, B, and E), and the outer cell mass (trophoblast) differentiates into the cytotrophoblast and syncytiotrophoblast. The syncytiotrophoblast invades the endometrial connective tissue 6-7 days after fertilization and starts secreting β-hCG, which signals to the corpus luteum in the ovary to continue producing progesterone. Accordingly, β-hCG may appear in the maternal serum 6 days after fertilization at the earliest and may require additional time to rise to a detectable level in serum (typically 8 days). β-hCG is detectable in the maternal serum at <5 IU/L, making serum β-hCG testing the most sensitive method of detecting pregnancy. (Choice D) A urine pregnancy test is less sensitive than a serum pregnancy test; it becomes positive when β-hCG reaches 20 IU/L, which typically occurs 14 days following fertilization. Educational objective: β-hCG is produced by the syncytiotrophoblast after implantation, which generally occurs 6-7 days after fertilization at the earliest. β-hCG typically is detectable in the maternal serum approximately 8 days after fertilization, whereas it is detectable in the urine 14 days after fertilization. Therefore, a serum pregnancy test will be positive before a urine pregnancy test. __________ References: Human chorionic gonadotropin in pregnancy diagnostics. (http://www.ncbi.nlm.nih.gov/pubmed/21635878)

A 41-Year-Old Woman, Gravida 0, Comes to the Office for Evaluation