"Most people do not understand what an odds game pregnancy is," says infertility specialist Dr. Sherman J. Silber.

Unmarried teen-agers who practice the "it-can't happen-to-me" method of birth control do not understand that they run a 40 percent chance of pregnancy each month.

Unaware of their odds, older couples who abandon birth control may vainly expect a pregnancy in a month or two. The chance of pregnancy for couples in their late 30s is only 8 percent per month, with only 65 percent of fertile couples achieving pregnancy after a year. Women in their late 30s are only one-third as fertile as women in their early 30s.

One out of six couples is infertile, about 10 million couples in the United States," says Silber, author of the tongue-in-cheek titled book, "How To Get Pregnant" (Scribners, 229 pages, $12.50). "Another one out of six will have difficulty in conceiving at some time in their childbearing years."

It is to this audience and to those interested in the prospects for treating infertility that Silber addresses his book.

Most doctors define infertility as the inability to conceive after a year or more of regular sexual relations without contraception, or as the inability to carry pregnancies to term.

Infertility, says Silber, seems to be on the increase. Contributing factors include the increased incidence of venereal disease and other infections, as well as the trend toward later marriage and childbearing. Fertility declines with age and delay in childbearing provides more time for infertility problems to occur, or worsen.

(More than half of infertile couples can achieve pregnancy with medical care, compared to about 5 percent spontaneous cure rate. And -- contrary to popular opinion -- applying for adoption has no effect on the percentages. Doctors are now able to diagnose the causes of infertility in more than 90 percent of the patients they see. In the recent past, doctors could find no cause in one out of four couples -- and were apt to give such folklore advice as a glass of sherry before-bedtime.)

Silber, who practices in St. Louis, comes to his interest in infertility by way of his skills as urologist and surgeon. In his early work in immunology and transplants, he practiced on rats, using microsurgery, a recent technique using microscope, delicate instruments, and thread invisible to the naked eye.

He later applied these techniques to reversing tubal ligations (sterilizations) in women, and in restoring normal sperm count in men who have had vasectomies.

A small number of microsurgeons has recently been able to achieve pregnancy rates of up to 80 percent in tubal-ligation reversal, and 60 percent or better in vasectomy reversal. Dr. Silber warns, however, that because of the intricacy of the microsurgery, sterilizations should still be considered irreversible.

Silber in his book wants to reassure couples of normal fertility, or with mild problems, who may suffer needless anxiety about their ability to conceive. m

"My goal," he says, "is to educate couples in detail about reproduction and the causes and treatment of infertility, so that they will know when and how to find an appropriate doctor and become partners with that doctor in their treatment."

Silber maintains that in nearly all cases, infertility is due to a relative deficiency in both the man and the woman, rather than a major deficiency in one partner. "A thorough evaluation of both," he says, "is the only way to maximize the fertility potential of the couple."

He offers this dramatic case history as illustration:

In 1977 Silber made medical history by performing a testicle transplant from Terry Twomey to his identical twin brother, Tim, who had been born without testicles. The operation was asuccess; Tim was able to produce sperm and to forego the monthly testosterone injections he had taken since age 18.

Two years passed and Tim's wife, Janice, had still not conceived. Her original evaluation as normal remained unquestioned in what was considered a one-sided case of infertility.

Finally, Dr. Emil Steinberger of the University of Texas, a pioneer in infertility research, discovered a slight hormonal imbalance -- an elevated testosterone level -- in Janice. Easily treated, Janice became pregnant within a few months and gave birth to a baby boy in March, 1980.