Reproductive Technology

Pregnancy and Infertility

There are many causes of infertility. Abnormal semen causes the infertility problems of about 30 percent of couples seeking treatment. Tubal disease and endometriosis in the female partner account for another 30 percent. A female partner's failure to ovulate accounts for 15 percent, and the inability of sperm to penetrate the woman's cervical mucus accounts for another 10 percent. The final 15 percent of couples seeking treatment are infertile for reasons that cannot be diagnosed.

Many couples can be helped to overcome infertility through hormonal or surgical interventions. Women experiencing ovulation disorders may benefit from treatment with oral drugs such as clomiphene citrate, or through the injection of gonadotropins, such as follicle-stimulating hormone, which has about a 75 percent success rate. Women with tubal disease can be helped by various types of reconstructive surgery, but the success rate is only about 33 percent.

However, many infertile couples cannot be helped by such standard methods of treatment. Instead, as a last resort, couples that want children must turn to newer techniques that bypass one or more steps in the usual physiological processes of ovulation, fertilization, and implantation. Commonly referred to as "assisted reproductive technology," these techniques include in vitro fertilization (IVF), gamete intrafallopian transfer, zygote intrafallopian transfer, donor insemination, egg donation, embryo cryopreservation, intracytoplasmic sperm injection, tubal embryo stage transfer, and intrauterine insemination.

In vitro Fertilization

When performed by an experienced practitioner and in an experienced clinic, IVF generally results in pregnancy rates of about 28 percent after one attempt and 51 percent after three. One study has reported the pregnancy rate after six attempts as being 56 percent. Another has reported it as being 66 percent.

Generally, one attempt at IVF is made per menstrual cycle. The IVF process begins when couples are first screened. Clinicians first must rule out infertility in the male partner. If the problem is with the female partner, various courses of treatment may be available. Generally, couples are expected to try to achieve pregnancy for a year after the initial screening before intervention is attempted. However, if a woman is in her late thirties or older, or if she is experiencing irregular menstruation, a clinical investigation may begin earlier.

Especially in older women, the blood level of follicle-stimulating hormone, a hormone that acts on the ovary to stimulate the maturation of viable eggs, is measured. If the hormone's level is found to be elevated early in a woman's menstrual cycle (after the first week of the new cycle), her ovaries may not be responding to it. In that case, hormonal treatment to stimulate ovulation would be ineffective, and assisted reproductive technology would be unable to help achieve pregnancy. Elevated estrogen levels at day three would also indicate that the ovaries are not responding correctly to estrogen or hormones.

In women whose ovaries are capable of generating viable eggs, the first step in IVF is referred to as "superovulation." To increase the chance of success, the woman's ovaries are stimulated to develop many follicles. Normally, only one or perhaps two follicles develop and are released by an ovary during a single menstrual cycle, which is why usually only one or, on rare occasions, two children are born. In superovulation, a doctor forces multiple follicles to develop so that many oocytes can be collected.

To stimulate the ovaries to develop many follicles, the woman undergoes the "long protocol." The action of the pituitary gland is suppressed hormonally, and ten days later the woman is treated with follicle-stimulating hormone. To see how well her ovaries are responding to the hormone, doctors measure estrogen blood levels and observe the ovaries with ultrasound scans. The number and size of the follicles can be visualized. When the doctors judge that the time is right (that is, when the follicle is enlarged to the point that it protrudes above the surface of the ovary), they give the woman human chorionic gonadotropin, wait thirty-six hours, and collect the oocytes from the mature follicles.

In the past, to collect follicles, doctors performed laparoscopy, in which a thin optical tube with a light (called a laproscope) is inserted through a very small incision in the abdominal wall, and the pelvic organs are viewed with fiber optics. Today, the use of a needle guided by ultrasound makes the procedure much faster. The ovary is visualized, mature follicles are located, the needle is inserted, and the follicular fluid that contains the mature oocyte (the unreleased egg) is aspirated. The doctors may collect up to eleven oocytes from a single patient.

Viable sperm are collected from the man and washed in a special solution that activates them so they can fertilize the egg. The process of sperm activation is called "capacitation" and normally occurs when sperm are ejaculated and enter the female reproductive tract. Capacitation involves activating enzymes in the sperm's acrosomal cap, allowing the sperm head, which contains the sperm's genetic material, to penetrate the outer and inner membranes of the egg (zona pellucida and vitelline membrane). For males with azoospermia, microsurgical or aspiration techniques can directly extract sperm from either the epididymis or the testicles. Azoospermia is the most severe form of male infertility, caused by obstructions in the genital tract or by testicular failure.

To allow fertilization to take place, a single egg and about 100,000 sperm are placed together in special culture medium and incubated for about twenty-four hours. Doctors then use a microscope to see if there are two pronuclei (one from the egg and one from the sperm) in the egg, indicating that fertilization occurred. In some cases, the sperm are unable to penetrate the egg. They may be unable to swim correctly, or they may not have been capacitated successfully. In the past, the only solution was to use sperm from another man. Now, however, sperm can be injected directly into the egg's cytoplasm, in a process called microassisted fertilization.

There are three ways used successfully by doctors and researchers to micro-fertilize the egg. The first is "zona drilling," in which a hole is punched into the zona pellucida, letting sperm penetrate the egg. The second method is called "subzonal sperm insertion," in which a sperm is injected directly under the zona pellucida. A third, related method is "intracytoplasmic sperm injection," in which a sperm is injected directly into the egg cytoplasm. The second method is reported to have a higher fertilization success rate (59%) than the third (13%).

The Risks of IVF

In some women, the drugs used to promote superovulation may cause side effects, including mood swings. Some investigators have suggested that procedures used in assisted reproductive technology may not be safe because of the potential for increased ovarian hyperstimulation syndrome and bone loss from the hormone treatments. Ovarian hyperstimulation syndrome occurs at mild levels in 23 percent of women undergoing the treatments, at moderate levels in 3 percent, and at severe levels in 0.1 percent. Complications also may arise as a result of the surgical procedures involved in egg retrieval and embryo transfer. Such complications include pelvic and other infections, which occur in 0.15 percent to 1.2 percent of women, complications from anesthesia, which occur in 0.2 percent, and internal injuries, which occur in 0.38 percent. Although the incidence of such complications is low, every chemical or surgical intervention is associated with risks, and potential patients should be aware of this.

Other concerns regarding the long-term effects of assisted reproductive technology include the increased incidence of spontaneous abortions, which occur in 20 percent of women, and ectopic and heterotopic pregnancies, which occur in 5.5 and 1.2 percent of women, respectively. In heterotopic pregnancies, an embryo is implanted outside the uterus.

There have been conflicting reports as to whether there is a link between the use of fertility drugs and ovarian cancer. Overall, results from many studies do not seem to support such a connection. Some researchers have suggested that the use of the drugs may increase the risk of ovarian cancer later in life, but this is difficult to prove or disprove because the techniques have not been around long enough to assess long-term effects. There have been no reports of any increase in abnormalities in children born using micro-assisted fertilization, though critics have questioned whether the techniques might increase the incidence of such abnormalities.

Embryo Transfer Techniques

Once the egg is fertilized, the two pronuclei fuse to form the early embryo, or "zygote." The zygotes are placed in culture media, where they undergo cell division, cleaving repeatedly and passing through the two-cell, four-cell, and eight-cell stages. Within seventy-two hours, the zygote develops into the morula, the solid mass of blastomeres formed by the cleavage of the fertilized ovum (egg). After about three to five days in culture, the zygote has become a hollow ball of cells called the blastocyst. During normal embryogenesis, it is the blastocyst that is implanted in the endometrial lining of the uterus.

While the embryos are in culture, problems in their development may become apparent. After embryos with evident problems are discarded, one or more cultured embryos are transferred into the uterus, where, it is hoped, one will become implanted and develop into a healthy, full-term baby. Embryo transfer replaces the natural process in which the embryo passes down the fallopian tube and into the uterus, prior to implantation.

Although this transfer is relatively simple and often takes only a few minutes, the rate of successful implantation is low. Usually, two embryos are transferred, but still only one of five women become pregnant. Most doctors who perform IVF procedures adhere to the limit of two embryos per woman to minimize the risk of multiple pregnancies. In most cases, three embryos are transferred in women older than thirty-five. In the United Kingdom, it is illegal for an IVF doctor to transfer more than three embryos at a time into a woman.

A number of factors play a role in whether embryo transfer leads to a baby being born. Success rate is higher if embryo transfer takes place between forty-eight and seventy-two hours after oocyte collection. When more than one embryo is transferred at the same time, the success rate increases, but so does the chance for multiple pregnancies. As noted above, the maximum number transferred should never exceed three. Probably the single most important factor determining whether or not a successful embryo implantation will take place is the donated egg's age. Embryos formed from eggs donated by younger women have a higher implantation success rate than do embryos formed from eggs donated by older women. The age of the host uterus appears to have little or no effect on outcome.

Gamete Intrafallopian Transfer

An alternative to IVF and intrauterine embryo transfer is gamete intrafallopian transfer (GIFT), introduced more than twenty years ago. In this procedure, the egg and sperm are collected as they would be for IVF procedures. However, instead of allowing fertilization to take place in a culture dish, the egg and sperm are transferred surgically into the woman's fallopian tube. This allows fertilization to occur in the fallopian tube, just as occurs in a natural pregnancy. The transfer can only be performed in women with healthy and functional fallopian tubes, and the sperm used for fertilization must be completely normal and capable of swimming.

After the transfer is made, doctors have no way of knowing if normal fertilization actually takes place until an embryo has implanted in the uterine wall. However, the procedure's success rate (35%) is higher than the success rate for IVF. Another related technique is zygote intrafallopian transfer, in which the egg is fertilized in vitro and the zygote is transferred surgically into the fallopian tube. Other techniques include tubal embryo transfer, in which an embryo already undergoing cleavage is transferred.

Intrauterine Insemination

Intrauterine insemination is used when a couple's inability to conceive a child is caused by the sperm's inability to reach the egg. Sperm must move through the uterus and enter the fallopian tube before they can fertilize the egg. Anything that prevents the sperm from making this trip will block conception. Coital or ejaculatory disorders can limit the sperm's travels, sperm antibodies in the female reproductive tract can kill the sperm, and sperm may be unable to penetrate the cervical mucus.

To help the sperm reach the egg, the female is treated with human chorionic gonadotropin to induce multiple ovulation. The number of follicles that are induced is monitored by ultrasound. Washed sperm from the male partner are injected through the cervical opening, into the uterus. The pregnancy rate using this procedure is about 10 percent.

Donor Insemination and Egg Donation

Donor insemination is used when sperm are incapable of fertilizing the egg. Usually this occurs if the male produces very little or no sperm. Sometimes, donor sperm is used when the male partner is the carrier of a genetic disorder that could be transmitted to the baby. Sperm donors should be between ages eighteen and fifty-five, and all should be screened for genetic disorders, such as cystic fibrosis, and for various types of chromosomal abnormalities and infectious disease, including hepatitis, syphilis, cytomegalovirus, and HIV. As with the use of intrauterine insemination, the female partner undergoes ovarian stimulation to maximize the number of follicles released during ovulation. Pregnancy rates resulting from the use of donor insemination are between 32 percent and 50 percent after ten inseminations.

As with donor insemination, egg donation is used when the woman cannot ovulate or is the carrier of a genetic disease. Egg donors must be younger than thirty-five years and must be screened for the same set of conditions as sperm donors. Donors are treated with drugs to stimulate ovulation, after which the eggs are fertilized with the sperm from the male partner and the embryos are transferred to the uterus of the female partner (other procedures can also be used). Growth and development of the embryos then follow the natural processes.

Surrogacy and Cryopreservation

Surrogacy, in which pregnancy occurs in another woman, can supply a couple with an alternative if the woman partner cannot carry the baby to term in her own uterus. In some cases, if the woman cannot supply the egg, sperm from the male partner can be used to inseminate the surrogate mother, who carries the baby to term. Alternatively, if the female partner can produce her own egg, sperm from the male partner can be used to fertilize the egg, and the resulting pre-embryo can be transferred to the uterus of the surrogate mother to grow and develop. Legal controversies resulting from these arrangements have become common in the last few years, so the arrangements should be carefully reviewed by all parties, along with experts in the field, before any final decisions are made.

Frozen sperm and embryos effectively retain their viability for many years. The use of frozen human blastocysts is associated with a 10 percent successful pregnancy rate. Oocyte freezing is much less successful, possibly because oocytes may be genetically damaged or killed in the freezing and thawing. Embryos produced from such cryopreserved eggs have a high incidence of aneuploidy, and they are slow to cleave and develop even if they appear to be genetically undamaged. Various research groups are trying to solve this problem.

Age as a Factor

Age must be taken into account when couples are considering assisted reproductive technology. In humans, the age of the oocyte, not the age of the uterus, is the main cause of reproductive failure in IVF and embryo transfer techniques. Embryos formed from older oocytes demonstrate an increased incidence of aneuploidy. In some other species, such as in rabbits, an aging uterus can keep an embryo from being implanted. The use of cryopreservation to circumvent reproductive failure in humans, cattle, and horses has already been successfully employed and is likely to be developed further.

Sperm generated by older men are capable of successfully producing normal embryos. However, as sperm age, they are exponentially more likely to contain new gene mutations. Older oocytes, on the other hand, do not appear to be more likely to contain new mutations. Scientists are unsure exactly how age affects oocyte integrity. Oocyte maturation takes place only before birth in the female, so no new oocytes are produced during the entire reproductive life of the female. This is quite different from spermatogenesis, which can continue into old age. Thus, oocytes from older women may be forty or more years old when they are collected and used to form the embryos.

Oocytes must reach full maturity before they can be ovulated normally and before they can be fertilized, even artificially, to form embryos. If immature oocytes could be artificially forced to mature in vitro, follicles could be taken from the ovaries of dying or dead women, or from cancer patients planning on undergoing chemotherapy treatments, which can damage oocytes. Unlike immature oocytes, immature sperm can effectively be used in fertilization. Additional research is needed in this area of assisted reproductive technology.

Legal, Ethical, and Moral Considerations

The use of these powerful techniques to facilitate reproduction in both humans and animals (the techniques can be used in cattle and pigs, and in the conservation of endangered wildlife) must be balanced against legal, ethical, and moral concerns. For example, would it be permissible to revive extinct animal species? Although a Jurassic Park-like scenario to reanimate extinct dinosaurs is not scientifically credible at this time, what if it became possible to use this technology to form embryos and clone an extinct mammoth, or the passenger pigeon? And what if we can do this for extinct humans? Just because we can develop the capability, would it be acceptable? What are the ethics involved?

Other concerns include questions about how long embryos should remain frozen and who owns frozen embryos not used by the parents. What happens if the parents separate, divorce, or die? What about the legal entanglements involved with surrogacy? Already in the media there have been a number of such cases reported. With the expected increase of these procedures in the future, it is likely that such complex questions will only escalate. Finally, there are basic concerns about helping people sidestep the natural birth process to bring into the world a new human.

SEE ALSO CELL CYCLE; CHROMOSOMAL ABERRATIONS; CLONING ORGANISMS; FERTILIZATION; LEGAL ISSUES; MEIOSIS; PRENATAL DIAGNOSIS; REPRODUCTIVE TECHNOLOGY: ETHICAL ISSUES.

Charles J. Grossman

and Robert Baumiller

Bibliography

Ryan, Michael. "Countdown to a Baby." New Yorker (July 21, 2002): 68-77.

Schultz, Richard M., and Carmen J. Williams. "The Science of ART." Science 21 (June 2002): 2188-2190.