A human egg is fertilized in the fallopian tube, specifically in the wide section closest to the ovary called the ampulla. This is the longest and widest part of the tube, and it’s where the egg and sperm typically meet within hours of ovulation. From there, the fertilized egg begins a multi-day journey down the tube toward the uterus, where it will eventually implant and grow.
Why the Ampulla Is the Fertilization Site
Each fallopian tube has several distinct sections. The ampulla makes up roughly the outer two-thirds of the tube and curves around the ovary. When an ovary releases an egg, finger-like projections at the end of the tube (called fimbriae) sweep the egg into the ampulla. Meanwhile, sperm that have traveled up from the uterus are already waiting in this region or arrive shortly after.
The ampulla provides the right environment for fertilization: it’s spacious enough for the egg and sperm to meet, and its inner lining secretes fluids that nourish and support both cells. In rare cases, fertilization may happen outside the tube entirely, in the abdominal cavity near the ovary, but the ampulla is the standard meeting point.
The Narrow Window for Fertilization
Timing matters more than most people realize. A released egg survives for less than 24 hours, and the highest pregnancy rates occur when sperm reach the egg within 4 to 6 hours of ovulation. Sperm, however, are more resilient. They can survive inside the reproductive tract for 3 to 5 days by binding to the lining of the fallopian tube, which keeps them alive and maintains their ability to fertilize. This is why intercourse in the days before ovulation can still lead to pregnancy: sperm may already be stationed in the tube, ready when the egg arrives.
How Sperm Penetrates the Egg
The egg is surrounded by a protective outer shell. When a sperm reaches this shell, it undergoes a chemical reaction at its tip that releases enzymes. These enzymes, combined with the physical force of the sperm’s movement, allow it to burrow through the outer layer. Once a single sperm breaks through and fuses with the egg’s inner membrane, the egg immediately changes its outer shell to block additional sperm from entering. This prevents more than one sperm from fertilizing the same egg.
The entire fusion creates a single new cell called a zygote, which contains a complete set of genetic instructions: half from the egg, half from the sperm.
The Journey From Tube to Uterus
After fertilization, the zygote doesn’t stay in the fallopian tube. It begins dividing as it travels, splitting from one cell into two, then four, then more. The tube’s inner lining has tiny hair-like structures that gently push the growing cell cluster toward the uterus. The zygote enters the uterus within 3 to 5 days.
By the time it arrives, it has developed into a ball of roughly 70 to 100 cells called a blastocyst. It floats in the uterine cavity for another 2 to 4 days before attaching to the uterine lining. This attachment, called implantation, typically happens between days 6 and 10 after fertilization, during a specific stretch of the menstrual cycle (days 16 to 22 of a 28-day cycle) when the uterine lining is receptive. Only after successful implantation does a pregnancy truly begin.
When Fertilization Happens in the Wrong Place
Sometimes a fertilized egg implants outside the uterus. This is called an ectopic pregnancy, and more than 90% of the time it happens in the fallopian tube itself, most often in the ampulla. This makes sense given that the ampulla is where fertilization occurs: if the zygote gets stuck or implants too early, it stays right where it was conceived.
A fallopian tube can’t stretch to accommodate a growing embryo. In the vast majority of cases, the tube ruptures, causing internal bleeding and a medical emergency. Warning signs include sharp pain on one side of the lower abdomen, abnormal vaginal bleeding, and dizziness. Ectopic pregnancies cannot result in a viable pregnancy and always require treatment.
In rarer cases (around 5% of ectopics), the egg implants on an ovary. A very small number implant in the abdominal cavity or the cervix.
How IVF Changes the Location
In vitro fertilization bypasses the fallopian tube entirely. Eggs are collected directly from the ovaries and combined with sperm in a lab dish, or a single sperm is injected directly into each egg. Fertilization happens in a controlled laboratory environment rather than inside the body. After the resulting embryo develops for a few days, it’s placed directly into the uterus.
This process was originally developed for people whose fallopian tubes are damaged or blocked, since those conditions prevent the egg and sperm from ever meeting at the ampulla. By moving fertilization outside the body, IVF removes the tube from the equation altogether.