Fertilization typically occurs in the ampulla of the fallopian tube. Once the sperm penetrates the secondary oocyte, a zygote is formed. This single-celled zygote undergoes rapid mitotic divisions known as cleavage while traveling down the tube toward the uterine cavity.
By about the fourth day, the dividing cells form a solid ball of roughly 16 cells called a morula. Fluid soon enters the intercellular spaces of the morula, creating a fluid-filled cavity known as the blastocoele. At this stage, the conceptus is officially a blastocyst.
The blastocyst features two distinct cell populations:
For successful implantation to occur, the endometrium must undergo structural and biochemical transformations to reach a receptive state. This period of peak uterine receptivity is called the window of implantation. In a standard 28-day menstrual cycle, this window opens around cycle days 20 to 24, corresponding to 6 to 10 days after ovulation.
Under the influence of progesterone, the endometrial stroma edematous changes occur, and spiral arteries coil extensively. The surface epithelium develops specialized pinopodes—small, microprotuberances that absorb uterine fluid and bring the blastocyst into close contact with the mucosal lining. If the embryo arrives too early or too late, the endometrium will be refractory, and implantation fails.
The physical process of blastocyst implantation unfolds in three sequential phases: apposition, adhesion, and invasion.
Successful implantation relies on a delicate balance of ovarian hormones. Estrogen promotes endometrial proliferation, while progesterone induces differentiation into a secretory, nutrient-rich bed.
As the blastocyst burrows in, the surrounding endometrial stromal cells undergo decidualization. These cells enlarge, accumulate glycogen and lipids, and transform into decidual cells. This decidual reaction creates a privileged nutritional matrix for the embryo and restrains excessive trophoblast invasion. Simultaneously, the developing syncytiotrophoblast secretes human chorionic gonadotropin, rescuing the corpus luteum from regression so it can continue producing progesterone until the placenta is fully functional.
When implantation occurs outside the normal uterine cavity, it is termed an ectopic pregnancy. The most common site for an ectopic pregnancy is the fallopian tube, specifically the ampulla. Tubal walls cannot support the growing trophoblast, leading to dangerous internal hemorrhage if rupture occurs.
Another complication is placenta previa, where the blastocyst implants in the lower uterine segment near or covering the cervical os. This condition typically presents with painless bright red vaginal bleeding in the third trimester. A common clinical misconception is that ectopic pregnancies can be salvaged and relocated into the uterus; once implantation occurs in an extrauterine site, termination or surgical removal is medically required to protect maternal life.