This article comes from an Ask the Expert conversation with Dr. Jill Corey, N.D. (they/them). For Them sourced questions from the community to ask to a trans healthcare professional.

For Them: When it comes to fertility in trans people on hormones, is there a known hormone level or other marker to determine current fertility? Generally, how can a trans person on hormones determine if they are capable of getting somebody pregnant or are capable of becoming pregnant (if they have not had any surgery on that part of the body)?

Dr. Jill Corey: There is no answer, unfortunately. The reason we can't say for sure is because every body is different. A certain level of hormones can do different things for different people. That's why when we initiate hormone therapy we have to make sure they understand hormone therapy is not a form of birth control or STI prevention. It is not a reliable method of birth control. You might not be able to have biological children or you may still be able to get someone pregnant or become pregnant.

Even with fertility testing, we still don't know for sure. For example, a person on testosterone but not menstruating could still be ovulating so there's a chance they could get pregnant by having sex with someone who has sperm. So, we have to treat everyone the same: if you don't want biological children, continue to use protection.

For Them: Can you talk more about taking birth control while taking testosterone? That can sometimes seem counterintuitive.

Dr. Jill Corey: Yes, it sounds counterintuitive to be on birth control while on T. We recommend the Mirena IUD that has a tiny bit of hormone that is localized in the uterus. So it doesn't necessarily go systemic and wouldn't interfere with testosterone therapy. The copper IUD (Paragard) will protect against pregnancy. If someone is not interested in getting an IUD, condoms are the best form of pregnancy prevention.

You could also do a permanent procedure called endometrial ablation. In this case, a surgeon goes in to remove all the endometrial tissue. You won't get your period, but you won't be able to have children -- it is permanent.

For preventing STIs, condoms are the best way to prevent STIs. Of course, you can still get herpes or HPV from skin to skin contact with condoms, but it is a low risk. PrEP, medicine to prevent HIV infection from sex or drug use, unfortunately hasn't been studied a whole lot in people with uteruses. There are two medications for use as PrEP. Truvada® is the only approved HIV prevention medicine for people with assigned female at birth. It takes 3 weeks for it to take effect. Descovy® has not been approved for AFAB people due to lack of research.