A Reproductive Endocrinologist (RE) is a doctor (M.D. or D.O.) who has completed specialized training in the field of medicine dealing with the hormones that govern conception. Sometimes, rather than calling them RE's, people refer to them as infertility specialists.
However, the term infertility specialist is a generic, unofficial term and can really apply to anybody who has a particular interest in fertility. Any family medicine doctor, general OB/Gyn doctor, acupuncturist, herbalist, nutritionist or other type of medical personnel can label themselves as a specialist in infertility.
In contrast, to consider oneself a reproductive endocrinologist, one must go through a very specific educational track. They must first graduate from college and attend four years of medical school. They then must get accepted into a residency in OB/Gyn where they will spend four years learning how to deliver babies, operate on the uterus and ovaries and take care of all aspects of women's health. During this time they will pass written examinations and then officially get their license to practice medicine. After these four years, they must apply to the limited number of accredited RE programs in the US. Once accepted, they will spend an additional three years focusing on learning infertility diagnosis and treatment.
The typical work week consists of meeting with patients to discuss their fertility issues, performing office procedures such as ultrasounds and inseminations, performing surgery such as laparoscopy and hysteroscopy and performing ART procedures such as egg retrievals and embryo transfers, all for the purpose of helping couples have babies.
While there is no clear shortage of RE's, the demand for their services is great and continues to increase as society displays a trend to delay childbearing until older age when it becomes more difficult. It is estimated that there are 1 million practicing physicians in the US. Out of those, there are about 1400 RE's or 1 out of every 700 doctors, making for a very limited subspecialty.