blastocyst

**Development timeline** - Day 0: Fertilization in the ampulla of the fallopian tube -> zygote. - Days 1-3: Cleavage divisions -> 2-, 4-, 8-cell stages (blastomeres). - Day 3-4: Morula (solid ball of ~16 cells) enters the uterine cavity. - Day 4-5: Fluid enters the morula (blastocoel forms) -> blastocyst. - Day 5-6: Blastocyst "hatches" from the zona pellucida (necessary for implantation). - Day 6-7: Implantation begins, typically in the posterior superior wall of the uterus. Occurs during the luteal (secretory) phase. - Week 2: Bilaminar disc (epiblast + hypoblast) forms. Week 3: Gastrulation -> trilaminar disc.

**Structure** - Trophoblast (outer cell mass): Forms the placenta (chorion). Differentiates into: - Cytotrophoblast (inner, mitotically active, single nucleus). - Syncytiotrophoblast (outer, multinucleated, invasive). It secretes hCG, which maintains the corpus luteum (and therefore progesterone) until the placenta takes over at about 8-10 weeks. hCG is detectable in maternal blood ~8 days after fertilization (about 1 week post-ovulation) and in urine slightly later. - Inner cell mass (embryoblast): Gives rise to the embryo proper, the amnion, the yolk sac, and the allantois (extraembryonic mesoderm is derived from the epiblast/hypoblast). - Blastocoel: Fluid-filled cavity. - Zona pellucida: Must be shed ("hatching"). It prevents premature implantation, for example in the fallopian tube.

**Clinical correlations** - Ectopic pregnancy: Implantation outside the uterine cavity, most commonly in the ampulla of the fallopian tube. Risk factors include PID (scarring), prior ectopic pregnancy, tubal surgery, and endometriosis. Presents with abdominal pain, vaginal bleeding, and amenorrhea, with a positive hCG and no intrauterine pregnancy on ultrasound. Rupture is a surgical emergency. Treat with methotrexate if stable and unruptured, or surgery. - Placenta previa: Implantation in the lower uterine segment, covering the internal os. Causes painless third-trimester bleeding. - Placenta accreta/increta/percreta: Defective decidua basalis. Abnormally adherent or invasive placenta. Risk factors include prior C-section or uterine surgery. - Hydatidiform mole: Abnormal trophoblastic proliferation. Complete mole (46,XX, paternal origin, empty egg + sperm duplication) has "honeycomb/snowstorm" appearance and very high hCG. Partial mole (69,XXX or XXY; two sperm + one egg) has a fetal parts. - Twinning timing: - Day 0-4 (before the morula/blastocyst forms): Dichorionic/diamniotic (separate placentas). - Day 4-8 (split of the inner cell mass): Monochorionic/diamniotic. - Day 8-12: Monochorionic/monoamniotic. - Day 13+: Conjoined twins. - IVF: Blastocyst transfer (day 5) allows better embryo selection, and preimplantation genetic testing (PGT) is done by trophectoderm biopsy. Embryonic stem cells are derived from the inner cell mass. - IUDs (copper) are spermicidal and induce a sterile inflammatory response that hinders fertilization and implantation. Emergency contraception (levonorgestrel, ulipristal) mainly delays ovulation. - Implantation bleeding: Light spotting about 1 week after conception.

**High-yield pearls** - Blastocyst = trophoblast + embryoblast + blastocoel. - Implantation occurs ~6 days after fertilization (the blastocyst stage) and requires the zona pellucida to hatch. - Syncytiotrophoblast makes hCG (structurally similar to LH, FSH, TSH alpha subunit); the hCG beta subunit is specific. - Decidualization of the endometrium is driven by progesterone.