What Is an REI? Reproductive Endocrinologist and Infertility Specialist Explained

Struggling to conceive? About 1 in 5 Americans face fertility challenges. A reproductive endocrinologist (REI) is a physician trained to diagnose and treat complex reproductive issues, guiding you through personalized infertility treatments like IVF, surrogacy, and egg donation.

What Does a Reproductive Endocrinologist Do?

A reproductive endocrinologist and infertility specialist (often referred to as an REI or RE) is an OB/GYN who has completed an additional 2–3 years of specialized training in reproductive medicine, infertility, and assisted reproductive technologies. 

REIs diagnose and treat complex fertility issues, including hormonal disorders, PCOS, endometriosis, and recurrent pregnancy loss. They also support single parents, LGBTQ+ families, donor conception, and surrogacy, providing specialized services such as IVF, IUI, fertility preservation, and advanced reproductive surgeries.

What’s The Difference Between an REI and an OB/GYN?

While OB/GYNs provide general reproductive care—including exams, Pap tests, cancer and STI screenings, contraception counseling, routine surgeries, prenatal care, and labor and delivery—they focus on overall female reproductive health. In contrast, REIs focus on complex fertility challenges and offer targeted expertise to improve conception outcomes.

Common Services Provided by REIs Include:

• Diagnostic testing: hormone panels, ultrasounds, semen analysis, genetic screening

• Ovulation induction and intrauterine insemination (IUI) to support conception

• In vitro fertilization (IVF): ovarian stimulation, egg retrieval, lab fertilization, and embryo transfer

• Egg and sperm donation coordination

• Fertility preservation: egg or embryo freezing for medical or personal reasons

• Advanced reproductive surgeries: hysteroscopy and laparoscopy to correct structural or anatomical issues

• Personalized counseling and fertility planning

The Role of an REI in Surrogacy

A fertility expert is essential in the surrogacy journey, guiding both intended parents and gestational carriers through the medical process.

For intended parents, the REI:

• Health and safety screening: REIs verify that egg and sperm samples are healthy, test for infectious diseases, and order genetic carrier screening to lower the risk of passing on inherited conditions.

• Embryo creation: Using in vitro fertilization (IVF), the REI (often called a reproductive endocrinologist, or RE) combines genetic material from the intended parents or from donors to create embryos.

• Full-process oversight: The specialist manages egg retrieval, fertilization in the lab, embryo culture, and the final transfer of the selected embryo(s) into the gestational carrier’s uterus.

For gestational surrogates, the REI:

• Evaluates surrogate’s overall health, reproductive history, and uterine readiness to confirm eligibility.

• Manages the surrogate IVF process—administering hormones to align her cycle for embryo transfer.

• Oversees the full medical arc—from initial screening through implantation and early pregnancy—turning embryos into healthy pregnancies and providing continuous support until care is transferred to the surrogate’s OB/GYN.

In short, the REI orchestrates the entire process—safely creating the embryo, timing its transfer perfectly, and guiding early pregnancy until it’s thriving.

When to See an REI

The American Society for Reproductive Medicine (ASRM) recommends consulting a fertility specialist if you experience difficulty conceiving or have certain medical considerations. For women under 35, it’s advised to seek help after 12 months of trying; for ages 35–39, after six months; and for those 40 and older, immediately upon deciding to conceive or after three months, as egg quality declines more rapidly.

Other indications include irregular or absent periods, uterine or tubal issues, severe endometriosis, a history of recurrent miscarriage, prior cancer treatments, or a strong family history of early menopause. 

For men, concerns may include testicular injury, prior surgeries, abnormal semen analysis, or severe male-factor infertility.

Fertility preservation, such as egg, sperm, or embryo freezing, is another reason to consult an expert. 

If you’re unsure whether it’s the right time, scheduling a consultation can provide clarity.

Choosing the Right REI and Fertility Clinic

The reasons you’re seeing a fertility specialist can play a big role in choosing the right fertility clinic. Your fertility history, the tests you’ll need, and the treatments you may require can make certain programs or doctors a better fit for your medical and personal needs. 

Finding a fertility specialist and clinic that align with your goals, offers the services you require, and makes you feel supported can improve both your experience and outcomes.

ConceiveAbilities: 30+ Years Building Modern Families

Reproductive endocrinologists and infertility specialists (REIs) are key partners in navigating infertility, surrogacy, and egg donation. Early consultation can improve outcomes, and seeking expert guidance is a proactive step that empowers your family-building journey.