In Vitro Fertilization 101

Process of spontaneous conception

Usually, a person releases one egg a month by a process called ovulation. During unprotected intercourse, the semen is released at the cervix. The sperm then travel through the cervical canal into the uterus and fallopian tubes where they wait for the egg to be released into the fallopian tube. Conception happens spontaneously when a sperm in the fallopian tube fertilizes an egg. An embryo is then formed, which travels through the tube back into the uterus and then implants. Because sperm can survive in the vagina and uterus for up to 7 days, intercourse 2 to 3 times a week would be sufficient for conception.

Rate of spontaneous conception

Conception in humans is not very efficient, and it may take several attempts over several months for conception to occur. Of all couples attempting to conceive, 15% will get pregnant per month and 85% will conceive within one year. Infertility is diagnosed when a couple does not conceive after one year. If above the age of 35 years, and with regular menstrual cycles, an evaluation after 6 months of actively trying to conceive with no success is recommended.

Causes of infertility

Couples may not conceive for many reasons. An explanation may be found equally among either partner, and sometimes more than one reason for infertility is found. Fifteen percent of the time, the reason for infertility remains unexplained. A common cause for infertility is either low sperm count (number) or low motility (percent of sperm moving). In both cases, very little sperm make it through the cervix into the uterus and the tubes, which makes the chance for the egg to be fertilized by a sperm very low.

Other causes for infertility are not releasing an egg on a regular (monthly) basis, release of an egg that is low quality (this is mainly seen with advanced maternal age), or blocked and/or damaged fallopian tubes. If an egg is not released during the monthly ovulatory phase of the menstrual cycle, conception cannot occur. If the fallopian tubes are damaged or blocked, the ovulated egg will not meet the sperm in the fallopian tube to be fertilized. If the egg is of low quality due to advanced maternal age, it may not be fertilized or if it is fertilized, it may result in a chromosomally abnormal embryo and lead to miscarriage. Less common reasons for infertility include abnormalities of the uterine cavity (polyps, septum, fibroids or scars) that may interfere with implantation, or pelvic adhesions (from previous infection or surgery) that may prevent the fallopian tube from picking up the ovulated egg.

Basics of IVF

In vitro fertilization (IVF) is the union of the egg and the sperm outside the body. Over several days, the fertilized egg becomes an embryo and is nourished in an incubator before being transferred back into the uterus. If the embryo implants in the uterus, then pregnancy can be achieved. IVF can treat most causes of infertility except in the event of an abnormal uterine cavity, in which case surgery is usually indicated. Although IVF is not the only fertility treatment available, it has the highest success rate. The national average for conception after one IVF cycle is around 50% for people less than 35 years of age. IVF is the only fertility treatment indicated with blocked or damage tubes or a very low sperm count.

Stimulation of the ovaries

For IVF to be efficient, the ovaries need to produce multiple mature eggs per IVF cycle. To achieve this, fertility drugs are injected daily to make the ovaries grow several follicles, which may or may not contain an egg, at the same time. The response to the fertility drugs is monitored with ultrasounds that assess follicular growth and blood work to assess hormone levels every 2 to 3 days. The medication dose is adjusted according to the response of the ovaries. The typical course of injections is 10-12 days, during which time the follicles continue to grow.

Egg retrieval and fertilization

Once the follicles reach a certain size, the egg retrieval occurs to get the eggs from the mature follicles under ultrasound guidance at a fertility center and usually under anesthesia. The number of eggs retrieved per IVF cycle varies depending on the response to the treatment. The eggs that are retrieved from the follicles are fertilized with sperm the same day of retrieval. After approximately 18 hours, the number of eggs that were successfully fertilized is known. The fertilized eggs are then incubated and observed as they develop into embryos. At Day 5 or 6 after the egg retrieval, the number and quality of embryos that successfully develop is assessed.

Embryo transfer

Prior to the embryo transfer, the embryo goes through several stages of development before it reaches the implantation stage (also known as the blastocyst stage). At this stage, certain cells in the embryo are already devoted to form the baby while other cells are designated to form the placenta. In vitro (in the culture medium) embryos reach the implantation stage (blastocyst stage) at 5 or 6 days of culture. Embryos can be transferred into the uterus at any stage of their development. However, most fertility centers grow the embryos to the implantation stage before the transfer. Transferring an embryo at the implantation stage will result in higher rate of successful implantation. This allows the transfer of less number of embryos into the uterus during the embryo transfer with a higher pregnancy rate. This practice has significantly lowered the incidence of multiple gestations (twins, triplets or more) resulting from IVF treatment. Currently, 1 embryo is typically transferred back into the uterus, and the remaining good quality embryos may be stored by cryopreservation (freezing) for future use.

The embryo transfer is a nonsurgical procedure that occurs at the fertility center in a procedure room while the woman is awake. The embryo(s) is placed in a special transfer catheter and the fertility specialist passes the catheter through the cervix into the uterine cavity. The transfer is done under ultrasound guidance.

Embryo cryopreservation

Cryopreservation involves freezing the embryos and storing them in liquid nitrogen. The chance of achieving a pregnancy from frozen embryo transfer is as good as with fresh embryo transfer. Once the embryos are cryopreserved (frozen), they can be stored for an indefinite length of time. Furthermore, embryos can be cryopreserved at any stage of their development. When the individual or couple wants to attempt pregnancy, the embryos are thawed. Few embryos (~ 10%) may not survive the freeze, or thawing process, and are lost. If the embryo was cryopreserved prior to the implantation stage, the embryos thawed and grown in culture medium to the implantation stage before they are transferred back into the uterus. Most fertility centers freeze the embryos at the implantation stage, in which case, the embryos are transferred the same day they are thawed.


Although the process of IVF may seem quite complicated and involved, advances in reproductive technology have allowed the process to become not only more efficient, but more successful. IVF is a valid option for many women who are having trouble conceiving on their own for any number of reasons. If you think IVF is the option for you, seek consultation with Reproductive Endocrinology and Infertility Specialist, or REI.

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