There are several ways fertilization can occur:

  • The natural way, via sexual intercourse between a man and a woman
  • Intracervical insemination, which is performed in a doctor’s office
  • Intrauterine insemination, also known as IUI and also done in a doctor’s office. This process involves transferring “washed” sperm directly into the uterus via a catheter
  • In vitro fertilization (IVF), a complex process provided at infertility clinics.

This article will describe, in general, these three methods of fertilization.

Fertilization the Natural Way

With sexual intercourse, intrauterine insemination, and IVF, in order for the egg to be fertilized, the head of the sperm must attach to the egg. As the sperm and egg merge, the contents of the sperm enter the egg, which causes the egg to immediately release a fluid called cytosol. Cytosol prevents other sperm from attaching themselves. Thus, after one sperm fertilizes the egg, no other sperm can penetrate the egg.

A normal human being has 23 pairs of chromosomes in every cell, or a total of 46 chromosomes. The sperm delivers 23 unpaired chromosomes, and the egg also contains 23 unpaired chromosomes. The nucleus of the sperm, which is now inside the egg, increases in size to form the male pronucleus, while the egg enlarges to form the female pronucleus.

The two nuclei then begin to form a complete set of 23 pairs of chromosomes. Once this happens, the fertilized egg now has 46 chromosomes; fertilization has occurred and implantation is set to take place.

Fertilization via Insemination (Intracervical and Intrauterine)

With insemination procedures the mother usually is given medication to stimulate multiple egg development, and the insemination is timed to coincide with ovulation. Insemination is a medical procedure that takes place in a doctor’s office.

The most important decision is the timing of the procedure. It should be done just before or just after ovulation has taken place. Once this has been established, the patient is prepared for a below-the-waist pelvic examination. A speculum is placed in the vagina, and the cervical area is gently cleaned.

If an IUI is done, the semen specimen is prepared ahead of time. The specimen is “washed,” creating a purified fraction of highly motile sperm. Using a sterile, thin, soft catheter, this specimen (perhaps just 1 cc of fluid or less) is placed inside the uterine cavity. This is quick and almost painless. Afterward, the patient may lie down on the exam table for a few minutes before getting dressed to leave.

If intracervical insemination is done, the semen does not need to be prepared. A special cup is placed in the vagina against the cervix. Semen is injected right into this cup, and then the opening used for the injection is plugged. The semen remains against the cervix for a while, and then the cup is removed.

Intrauterine insemination has a better success rate than intracervical insemination, and is therefore the preferred method at most fertility clinics. This is because the specimen has been prepared to contain the healthiest sperm, with a high percentage of “swimmers.” Also, the IUI introduces the sperm directly into the uterine cavity, closer to their target egg, which is making its way down the fallopian tube.

Intracervical insemination is closer to natural sex in terms of how fertilization takes place, because the sperm are still in the vagina and they have to swim through the cervix and uterine cavity to make their way to the tube—the same process that takes place with sexual intercourse.

Fertilization via In Vitro Fertilization

In vitro fertilization, also known as IVF, takes place in a laboratory at an infertility clinic. After much medication and monitoring, the reproductive endocrinologist retrieves the eggs from the mother or egg donor, and the embryologist oversees the fertilization process. IVF is the process of generating human life when other methods have failed.

The eggs (also referred to as ova) and sperm are mixed together until the viable eggs are fertilized and embryos are developed. Once fertilization has occurred, usually no more than two embryos are then transferred back into the mother or gestational carrier (surrogate) in hopes that at least one embryo will implant itself in the lining of the uterus.

IVF should be monitored closely by the doctor involved to ensure the safety of both the mother and the baby. At approximately 10–12 weeks, the pregnant woman is then released from the infertility clinic to the OB of her choice, and the pregnancy is treated like a normal occurrence.

Just as with normal pregnancy, early pregnancy after IVF may not progress as hoped. The fertilized egg may not implant at all. Or it may implant but stop growing, resulting in a miscarriage. There is also a small chance of ectopic or tubal pregnancy—since the transferred embryo is in fluid, it may float into a tube and then implant there. Also, a single transferred embryo will sometimes divide into two, becoming identical twins. Two for the price of one!

There are variations in how IVF is done. Sometimes it is a “fresh” cycle. This means that about three days after fertilization has taken place, the newly created embryos are evaluated and the best one or two are “transferred” back into the uterine cavity, a process similar to IUI above. The recipient has been carefully prepared with hormones to maximize the chance that implantation takes place.

Sometimes the recipient is a surrogate. As long as her body has been hormonally prepared ahead of time, the embryo can be transferred into her rather than into the biological mother from whom the eggs were retrieved. Or the eggs can be retrieved from an egg “donor,” and then three days later a fertilized egg can be transferred into the intended recipient (the client).

Sometimes the fresh embryos are genetically tested. This is called PGD (preimplantation genetic diagnosis). This can be extremely useful for many reasons. One is to make sure the embryo has viable DNA. This lowers the risk of failure of the cycle or subsequent miscarriage.

Sometimes the family really wants a child of a certain sex. This can also be done using PGD. Sometimes there is a genetic disease in the family, and the embryos can be tested to make sure the selected ones do not harbor this genetic condition. In the case of PGD, the embryo might be frozen while waiting for test results, and might not be transferred until it is five or even six days old.

Frozen embryo transfer is done more often than fresh. It is common for the egg retrieval process to yield many eggs that successfully fertilize. After three days, the best one or two are utilized for the fresh transfer and the rest are frozen. These can be thawed out months or even years in the future and still have a high chance of being successfully implanted.

A woman can give birth to a child years after the baby’s father has passed away, or perhaps after a divorce. This is why many court battles have been fought over the “custody” of frozen embryos. Most IVF programs require that both the biological mother and the biological father provide written consent for a frozen embryo to be used, and a contract specifies the conditions under which remaining frozen embryos are removed from the freezer and “destroyed.”

Male factor fertility can be treated with IVF also. Low sperm count is a common problem. Using microscopic needles and catheters, IVF labs can select a single healthy sperm and literally inject it directly into an egg, resulting in fertilization. This is called ICSI (intra-cytoplasmic sperm injection).

Men who have zero sperm in their semen can be helped using a procedure called PESA (percutaneous epididymal sperm aspiration). This when a needle is inserted into the testicle, and immature sperm that are not fully developed are extracted. They still contain the expected 23 chromosomes, but they cannot swim. PESA is followed by ICSI, and a fertilized egg can be produced, which is then used for IVF fresh or frozen.

[Page updated December 2015]