Intrauterine insemination (IUI) simply refers to the procedure where sperm are placed in the uterus. Eggs (oocytes) are fertilized in the fallopian tube. A normal ejaculate will have >20 million total motile sperm but of this number, when having intercourse, only as few as a couple of hundred to a few thousand make it to the tube where the egg needs to be fertilized. So, placing sperm beyond the major filter, which is the cervical mucus, tremendously increases the number of sperm in the fallopian tube.
Sperm need to be “washed” before they’re ready for IUI. Washing must be done because the unwashed semen specimen contains not only the sperm, but also the seminal plasma. This liquid, which originates in the prostate gland, is designed to protect the sperm from the acidic environment of the vagina. It is alkaline, forms a sticky clump to keep the sperm in the vagina, and contains large amounts of prostaglandins, chemicals that cause smooth muscle contractions. None of these qualities are favorable for a trip directly into the uterine cavity. If a large amount of semen — more than 0.2 ml — is injected unwashed into the uterus, the prostaglandins can cause severe cramping at the least!
In preparing the sperm the semen specimen is first diluted with culture media (a sperm nutrient solution) and the sperm are spun down into a little pellet in a centrifuge before being placed in the uterus. One method spins the sperm in a centrifuge, and another puts the sperm on the bottom of a test tube and allows the best sperm to swim up, and third method combines the two: centrifugation followed by swim-up.
Another common method used puts the sperm on top of several layers of washing media; the tube is spun down, and the pellet on the bottom will contain the largest amount of motile, healthy sperm. The sperm pellet is then placed via catheter into the uterus. All methods have in common the concept of isolating the sperm from the seminal plasma. Recently sperm separation devices have been introduced based upon microfluidics and claim to isolate sperm that have less DNA damage.
Two points about the IUI process: it does not make better sperm and it does not increase the total number of motile sperm. Sperm are either structurally normal and have normal fertility or they don’t. Isolating damaged sperm will not make them better. After the sperm are washed, only the motile sperm are collected, and this can give a false sense that somehow immotile sperm were made to move. Don’t be misled if the post-wash specimen has a motility that is significantly higher than the pre-wash specimen. After all, if all you are collecting are motile sperm, then the percent of motile sperm should be high –the total number of sperm, however, will be lower.
The main risk of doing IUI is the risk of infection which is quite low. Estimates of infection rates are usually < 1%.