A baby starts growing in the fallopian tube, not the uterus. Fertilization happens in the wider section of the fallopian tube called the ampulla, where a sperm meets and penetrates the egg. From there, the fertilized egg spends several days traveling toward the uterus, where it will settle in and grow for the remainder of pregnancy.
Fertilization Happens in the Fallopian Tube
Each fallopian tube connects an ovary to the uterus, and the ampulla is the widest part of the tube, located closest to the ovary. After ovulation releases an egg, it enters the fallopian tube and waits in this section. If sperm are present, fertilization occurs here. The moment a sperm successfully enters the egg, the outer shell of the egg hardens to block other sperm, and the combined genetic material forms a single cell called a zygote.
That single cell immediately begins dividing. Within hours it becomes two cells, then four, then eight. All of this early division happens while the tiny cluster of cells is still inside the fallopian tube, slowly being pushed toward the uterus by rhythmic contractions and tiny hair-like structures lining the tube walls.
The Five-Day Trip to the Uterus
It takes roughly five to six days after fertilization for the growing cell cluster to reach the uterus. By the time it arrives, it has divided dozens of times and organized itself into a hollow ball of about 200 to 300 cells called a blastocyst. This structure has two distinct parts: an outer layer that will eventually form the placenta and an inner cluster that will become the embryo itself.
The blastocyst sheds its protective outer shell (a protein coating it carried since it was an unfertilized egg) and is now ready to attach to the uterine wall. This shedding, sometimes called “hatching,” is a necessary step. Without it, the blastocyst can’t make direct contact with the uterine lining.
How the Embryo Attaches to the Uterine Wall
Implantation is the process that transforms a free-floating blastocyst into an actual pregnancy, and it’s far more involved than simply “sticking” to the wall. The uterine lining, called the endometrium, has to actively prepare for the embryo’s arrival. Hormones trigger the surface cells of the lining to develop smooth, bulging protrusions that strip away a protective mucus barrier normally covering the uterine wall. This creates a brief window, typically lasting only a few days, when the lining is receptive enough for an embryo to attach.
Once the blastocyst makes contact, it doesn’t just sit on the surface. It burrows into the lining, penetrating through the outer layer and eventually tapping into the mother’s blood supply. This invasion is what allows the placenta to form later, creating the lifeline that delivers oxygen and nutrients throughout pregnancy. The endometrium plays a critical role here: specialized blood vessels called spiral arteries develop within it to supply the implantation site, and their growth is essential for sustaining the pregnancy.
The thickness of the uterine lining matters for this process. Research on fertility treatments has found that pregnancy rates drop when the lining measures less than 7 millimeters, with the best outcomes occurring when it reaches 10 to 12 millimeters. While this data comes from assisted reproduction, it illustrates how dependent implantation is on the lining being thick and well-supplied with blood.
What Happens After Implantation
Once embedded in the uterine wall, the developing organism is called an embryo. Organ formation begins remarkably early, starting in the third week after conception and continuing through the eighth week. The cardiovascular system is the first to develop. The heart forms its four chambers by just four weeks after conception, making it the first functioning organ in the body. The liver also appears early, beginning in week three and growing rapidly through week ten.
At nine weeks after conception (which corresponds to the 11th week of pregnancy, since doctors count from the last menstrual period), the embryo officially becomes a fetus. By this point, all major organ systems have at least begun to form, and the remaining months focus on growth and maturation rather than building new structures from scratch.
When Implantation Happens in the Wrong Place
In about 1% to 2% of pregnancies, the fertilized egg implants somewhere other than the uterus. This is called an ectopic pregnancy, and roughly 97% of them occur in the fallopian tube itself, most often in the same ampulla region where fertilization originally took place. The embryo essentially never completes its journey to the uterus.
Within the fallopian tube, about 70% of ectopic pregnancies lodge in the ampulla. The rest can occur along other sections of the tube. In rare cases, implantation happens in even more unusual locations: the ovary (fewer than 3% of ectopic cases), the cervix (fewer than 1%), the abdomen, or a previous cesarean scar. These pregnancies cannot develop normally because only the uterus has the blood supply, stretching capacity, and hormonal responsiveness needed to support a full pregnancy. Ectopic pregnancies require medical treatment, as a growing embryo in the fallopian tube can cause the tube to rupture, which happens in about 15% of tubal ectopic cases.
Risk factors for ectopic pregnancy include previous fallopian tube damage, prior ectopic pregnancies, and certain infections that cause scarring in the tubes. Symptoms typically include one-sided pelvic pain and vaginal bleeding in early pregnancy, often before a person even knows they’re pregnant.