Often people think about fertility as a "woman's issue," but actually the person trying to get pregnant is only half of the equation. As it turns out, fertility for the person with the penis (if the other person does have one, because heteronormative couples aren't the only couples trying to get pregnant) can be pretty complex, too. So if you're considering getting pregnant and your partner has only one testicle, you might worry and, subsequently, as yourself, "Can I get pregnant if my partner only has one ball?" According to experts it depends on the reason why your partner is down one testicle, and whether or not their remaining testicle is functional.
According to the Mayo Clinic, male fertility issues can be caused by a variety of factors. The clinic's website explains that to get a person pregnant, one needs to be able to produce healthy, mobile sperm and get that sperm to their partner's egg at the right time. Because having an undescended testicle, a condition where, according to The Mayo Clinic, one or both testicles didn't descend into the scrotum during fetal development, might impact sperm quality or quantity, it might increase your partner's chance of being infertile, if this was the cause as to why they don't have two working testicles.
If your partner's testicle was removed due to testicular cancer, his fertility might also be impacted in a number of ways. According to the American Cancer Society, both testicular cancer, as well as its treatment, can cause infertility issues by affecting hormone levels and lowering sperm counts. The society's website encourages people with testicular cancer to discuss options for preserving fertility with their doctor, including storing their sperm, before beginning the particular cancer treatment their health care providers have suggested. Fortunately, according to their website, infertility from cancer treatment won't necessarily be permanent if the patient retains one testicle, and fertility can actually return two years after chemotherapy ends.
According to Go Ask Alice, a team of health professionals at Columbia University, having one testicle might not automatically mean a man will suffer from infertility. The group writes in one answer to a visitor's question, "In most cases, more than enough hormones and sperm are produced in one healthy testicle for both a healthy sex life and the ability to fertilize eggs. This is similar to people who have one lung, kidney, or ovary; only one organ is truly necessary."
So while it might not necessarily impact your partner's ability to produce enough healthy, active sperm to facilitate conception, it's best to consult a health care provider if you have questions about your or your partner's fertility, or if you have been unable to conceive and want to find answers.