ovum, sperm, fertilization

How Long Does It Take Sperm to Reach the Egg After IUI?

After a man ejaculates, millions of sperm begin swimming upstream toward the egg waiting in your fallopian tubes. The journey is a complicated one and includes many natural barriers, like thick cervical mucus.

IUI is a fertility treatment that can help improve pregnancy chances for couples and individuals with unexplained infertility. During IUI, doctors collect and wash sperm so they are much closer to the egg.


During sexual intercourse, about 40 million to 150 million sperm are ejaculated and start swimming toward the fallopian tubes. This is a journey that can take up to 45 minutes for the fastest swimmers, or up to 12 hours for the slowest. It’s also a journey that’s littered with obstacles. Sperm encounter natural “booby traps,” like thick cervical mucus that’s almost impenetrable on non-fertile days but somehow loosens up during the most fertile time of a woman’s menstrual cycle.

The goal is to reach an egg and fertilize it to create a zygote. Fertilization happens when a single sperm successfully penetrates the outer layer of an egg (the zona pellucida) and fuses with its genetic material. During this process, the sperm’s nucleus and the egg’s pronucleus fuse to form a new cell that is then carried down the fallopian tube and into the uterus, where it becomes embedded in the uterine lining.

Before sperm can penetrate the egg, it must undergo a series of complex steps called capacitation and the acrosome reaction. Capacitation involves a change in the structure of the sperm’s surface proteins to allow it to adhere to and penetrate the egg. The acrosome reaction releases a chemical called hyaluronic acid, which breaks down the membrane surrounding the egg and allows sperm to pass through.

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IUI is performed near the time of ovulation to make it easier for sperm to reach the egg and overcome some of the natural barriers that limit fertility. It’s usually a first-line treatment approach for unexplained infertility, along with medication to induce ovulation and medications for endometriosis. During this procedure, the clinician uses a speculum to view a woman’s cervix and inserts a catheter into her uterus. Then, washed sperm is slowly injected into her uterus.

Embryo Development

Once the sperm reaches the egg, it must fertilize it. This is accomplished in a process called capacitation, where the sperm binds to the outer layer of the egg and a hardened layer of protein inside called zona pellucida that prevents other sperm from entering.

Once fertilized, the zygote (fertilized egg) travels down the fallopian tube until it reaches your uterus. It then attaches to the lining of your uterus, known as the endometrium, where it will continue to grow and develop.

It takes about a day for the zygote to reach the uterus from the fallopian tube. Once there, it takes about another three to five days for the cells of the zygote to continue dividing until it becomes a hollow ball of cells – called a blastocyst.

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During the embryonic stage, which lasts through the eighth week after fertilization, the fetus grows much bigger and changes in appearance and function. The organs of the body begin to take shape, including the heart and lungs. Grooves are forming in the embryo’s arms and legs, which will eventually become webbed feet and hands with digits. The eyes and ears are forming as well.

In order to be able to reach the egg, sperm must travel a long distance from the vagina through the cervix and fallopian tubes. Only about 5% of the sperm that is released during sexual activity is able to successfully reach the egg. With IUI, a sample of the sperm is collected, washed and concentrated so that only high-quality sperm remains. This sperm is then placed directly into the uterus using a catheter, cutting down on the amount of time and distance that the sperm has to travel. This also improves the chance that the sperm will fertilize an egg.


Sperm must race against time to reach an egg, and the odds are stacked against them. Many sperm die before they get close to an egg, and even the hardiest of them may have trouble penetrating the thick egg membrane. Once a sperm makes it inside, it must penetrate and fertilize the egg within 24 hours or it will die. The fertilized egg then leaves the fallopian tube and enters your uterus 3 to 4 days later.

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IUI is a simple procedure that only takes a minute or two and can be done in your doctor’s office or a clinic that specializes in fertility issues. Your healthcare provider will first determine the optimal time to perform IUI, usually a day or two after you ovulate. This can be done using an at-home ovulation predictor kit that detects the release of luteinizing hormone (LH), or your healthcare provider may use a transvaginal ultrasound to see signs of mature eggs.

During IUI, the sperm are washed and concentrated so that only high-quality sperm remain. They’re then injected into your uterus with a catheter, which bypasses your cervix and increases the chances of the sperm reaching an egg. You’ll likely be asked to lie down after the procedure for 10-15 minutes.

You’ll probably also be given a medication called progesterone after IUI, which helps maintain the uterus’s lining and improves the chances of implantation. You can buy this drug over-the-counter, and it’s also in most prenatal vitamins. Once a fertilized egg implants in your uterus, you’ll likely be given a blood test to confirm pregnancy. A positive result shows that the fertilized egg has made its way into your uterus and is growing into the embryo that will eventually become your baby. A negative result means the embryo didn’t implant, and you won’t be pregnant.