The Four Stages of Embryo Fertilization During In Vitro

A four cell embryo after in vitro fertilization

There are several stages of embryo fertilization during in vitro procedures.

In Vitro Fertilization

For some couples, achieving pregnancy is difficult even after multiple forms of conventional fertility treatments like fertility drugs, artificial insemination, or even surgical procedures to correct any physical issues. For these women, in vitro fertilization (IVF), which is the fertilization of an egg in a laboratory followed by transfer of the embryo into the uterus, may be the answer to conceiving. Success rates vary from 35 percent for women under age 35 to 10 percent for women over age 40.

Stages of Embryo Fertilization During In Vitro

It may take several cycles for a couple to have success with IVF. Each cycle of in vitro fertilization contains a number of steps and can take up to six weeks to complete. During these stages of embryo fertilization during in vitro, you will be in close contact with your reproductive specialist.

Follicular Development

During this stage of embryo fertilization, the woman is given fertility drugs to stimulate the ovaries to produce numerous healthy eggs that will be used in the procedure. The fertility drugs are usually given by injection at home by the woman or her partner for a period of 10-20 days.

The reproductive specialist will closely monitor the woman's progress during this time to determine if the ovaries are developing follicles that contain the eggs. Using blood tests and ultrasounds, the physician can follow the size of any developing follicles; once the follicles are ready to mature, another medication is given to complete the process.

Egg Retrieval

Once the follicles mature, the specialist will retrieve the eggs from the ovary while the woman is under sedation. Using an ultrasound, the physician can localize the follicle and aspirate the contents through the vagina. Occasionally, the fluid may have to be removed through a small incision in the abdomen. This fluid needs to be examined under a microscope to examine the eggs that are present and to make sure that there are enough eggs to continue with the procedure.


After the eggs have been examined, they will be fertilized. A semen sample, collected from the husband, partner, or donor, is then examined; the technician separates the sperm from the sample and looks at the sperm. If the sperm quality and number is normal, the egg and sperm are combined in a plastic dish. If there is some problem with the sperm like low numbers or low motility, the sperm may be injected into the egg in a process called intracytoplasmic sperm injection.

With either process, the inseminated eggs are placed in an incubator to encourage fertilization and the development of the embryos. Within a couple of days, the doctors hope to see a two to four cell embryo if the process has been successful.

Embryo Transfer

Once embryos have formed, the next stage of embryo fertilization during in vitro is the placement into the endometrial cavity. The doctor uses a thin catheter inserted through the cervix and a trans-abdominal ultrasound to place the embryos into the uterus.

The transfer can take place as soon as three days after fertilization; the embryo is in the cleavage stage at that point. In some cases, the doctor may wait until five or six days after fertilization to perform the transfer when the embryo is called a blastocyst.

The number of embryos that are transferred at one time depends on the age of the woman undergoing the procedure and the stage of the embryo. At the cleavage stage, more embryos would be transferred into an older woman while two may be transferred in women under 35. However, if the embryos are transferred at the blastocyst stage, the doctor may only transfer one or two at a time.

Pregnancy Testing

Two weeks after the embryo transfer, the physician will perform blood and urine tests to determine if a pregnancy has taken place. If the IVF has been successful, the reproductive physician will monitor the woman closely by performing repeat pregnancy tests and the status of the pregnancy but will also refer her to an obstetrician for prenatal care.

The stages of embryo fertilization during in vitro can result in extra embryos being created. These embryos can be frozen to be used at a later date if desired.

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The Four Stages of Embryo Fertilization During In Vitro