"Without these data, we were concerned that frozen sperm might reduce the birth rate," says Alan Thornhill, Ph.D., senior author of the study and director of the Mayo Clinic in vitro fertilization laboratory. "Now, we believe that concern is unwarranted."
In vitro fertilization starts with a woman taking fertility drugs to stimulate her ovaries to produce more eggs than usual. When the eggs are mature, they are retrieved from the ovary and introduced to washed sperm to allow fertilization. The availability of sperm on the day eggs are retrieved is critical to success.
Two to five days after retrieval, the embryos (the fertilized eggs) are transferred to the uterus.
How the study was done
Researchers reviewed data collected at Mayo Clinic from 1993 to 2003. Fresh sperm were used in 1,580 cycles and frozen sperm in 309 cycles. At Mayo Clinic, sperm samples are frozen as part of the in vitro process in the event that a fresh sample is not available on the required day.
Researchers compared the effectiveness of fresh vs. frozen sperm by calculating the cumulative live birth rate -- that is at least one baby born from a single egg retrieval from the mother. Embryos from a single egg retrieval may be transferred over one or more transfer cycles.
For cycles using fresh sperm, the cumulative live birth rate was 51.6 percent. For frozen, it was 53.1 percent.
Dr. Thornhill says Mayo Clinic doctors still prefer to use fresh sperm when possible because the number of sperm and their movement are reduced by freezing and thawing.
If fresh sperm is not available -- because of the man's illness, travel or other circumstances -- couples can choose to use frozen sperm knowing they aren't reducing their chance for a successful pregnancy.
"The in vitro process is long and can be difficult -- emotionally, physically and financially," says Dr. Thornhill. "These results make the process just a little bit easier."