A 29-year-old woman presents to the emergency department with severe left lower quadrant abdominal pain and light vaginal bleeding. Her last menstrual period was 7 weeks ago. She has a history of pelvic inflammatory disease treated several years earlier. Vital signs reveal a blood pressure of 92/58 mm Hg and a heart rate of 118/min. Serum β-hCG is positive, and transvaginal ultrasonography demonstrates an empty uterine cavity with a gestational sac located within the left fallopian tube.
Which of the following best explains the embryologic basis of this patient's condition?
The correct answer is:
B) Implantation occurred before the blastocyst reached the uterine cavity
Fertilization normally occurs in the ampulla of the fallopian tube. The resulting zygote undergoes cleavage while traveling toward the uterus, becoming a morula and then a blastocyst. Implantation normally begins 6–7 days after fertilization, when the blastocyst reaches the uterine cavity and the trophoblast invades the endometrium. Tubal damage from pelvic inflammatory disease can impair transport of the blastocyst, causing implantation to occur prematurely within the fallopian tube and resulting in an ectopic pregnancy.
Answer choice A: Fertilization normally occurs within the uterine cavity rather than the fallopian tube, is incorrect. Fertilization normally occurs in the ampulla of the fallopian tube, not in the uterus. The abnormality in this patient is the site of implantation, not fertilization.
Answer choice C: The morula normally implants within the ampulla of the fallopian tube, is incorrect. The morula is a preimplantation stage that remains enclosed by the zona pellucida. Implantation is initiated by the blastocyst, not the morula.
Answer choice D: The primary oocyte implanted before completion of meiosis I, is incorrect. Implantation involves the blastocyst several days after fertilization. The primary oocyte completes meiosis I immediately before ovulation, and the secondary oocyte completes meiosis II only after fertilization.
Answer choice E: The trophoblast failed to differentiate into a syncytiotrophoblast, is incorrect. Differentiation into the syncytiotrophoblast is required for implantation and does occur in ectopic pregnancies. In fact, the syncytiotrophoblast secretes β-hCG, explaining this patient's positive pregnancy test.
Key Learning Point
Ectopic pregnancy most commonly results from implantation of the blastocyst within the fallopian tube due to impaired tubal transport, often caused by prior pelvic inflammatory disease. Fertilization occurs normally in the ampulla, but implantation should occur in the uterine endometrium approximately 6–7 days later.
