the very first...

I think it's most fitting to start with the fantastic legend of Agnodice. Though it is largely doubted and little known, the legend was once relied upon by women who were forbidden to enter the medical profession. Here's the story:

In the 4th Century BCE there lived a woman named Agnodice who wished to practice medicine in Greece. All females and slaves were never permitted to legally hold this position, but Agnodice had such a strong desire that she found a way around the law. By cutting her hair and wearing men's clothing she fooled the well known doctor Herophilus and went about her training. One day, she heard the moans of a woman in labor. When Agnodice approached, she was told to leave her be because she appeared to be a man. Agnodice knew she could help if she could only prove she was a woman too, so she lifted her garments. The woman then let the new doctor catch her baby, and with great success. Agnodice soon became well known amongst the women of Greece because of her tender ways and understanding. Her acclaim brought her much attention from the other doctors of Greece. Out of suspicion and jealousy, they accused her of seducing patients and took her to court. At the trial, many women who Agnodice had treated came to offer support. They spoke the truth about Agnodice and said they would choose her above any male doctor to treat them. Because of the overwhelming consensus of the women, the courts amended the law and allowed females to become medical doctors. Agnodice was then known as the first female doctor, specifically a midwife.

Hooray! Except, the legend is now thought to be false. The only account is based on the writings of Hyginus, and there are discrepancies about the facts. But, it's a lovely legend all the same and has allowed many women since enter the medical field.