Fertilization and the First Days of Pregnancy

The Story

Fertilization normally happens in the fallopian tube. After ovulationThe release of a mature oocyte from the ovary., the fimbriaeFinger-like projections of the uterine tube that help capture the ovulated oocyte. sweep the egg into the tube; sperm travel up to meet it; one sperm penetrates, the egg completes meiosis, and the two haploid nucleiClusters of neurons in the CNS responsible for processing information. fuse into a diploidA cell with two sets of chromosomes, one from each parent (2n). zygote. As the zygote drifts down the tube toward the uterusThe muscular organ where a fertilized egg implants and develops. over several days, it divides — 2 cellsThe basic structural and functional units of life., 4, 8 — becoming a morula and then a fluid-filled blastocyst. Around day six or seven the blastocyst reaches the progesterone-prepared, secretory endometriumThe inner lining of the uterus that thickens during the menstrual cycle to support a potential pregn and implants. The implanting tissue secretes hCG, which rescues the corpus luteum so progesteroneA hormone that supports pregnancy and regulates the menstrual cycle. keeps the lining intact, and early pregnancy is underway.

Read that sequence with Stina’s anatomyThe study of the structure of the human body. in mind and the failure points light up. The egg must be captured and carried by a patent, mobile fallopian tube — the very structure most vulnerable to external scarring. The endometrium must be properly primed by a clean progesterone signal — the very signal a disrupted cycle blurs. A blocked or kinked tube can prevent the egg and sperm from ever meeting, or trap a dividing embryo in the tube (an ectopic risk); a poorly prepared lining can fail to support implantation. Infertility, in other words, is rarely ‘nothing wrong’ — it is a precisely located stall in a precisely timed process. The next page names what, in Stina, was doing the stalling.

From Stina’s chart: For Stina, this is the page that never happened — and understanding exactly where the process stalls is what turns ‘unexplained’ infertility into something nameable and treatable.