The Corpus Luteum, Progesterone, and Pregnancy

In the 1920s, scientists found estrogen and learned about its impact on female reproductive function. Some scientists thought estrogen was the only female sex hormone. However, some researchers were not convinced. In 1897, John Beard suggested that the corpus luteum, which is the remains of a follicle after ovulation, could be an important part of pregnancy. He theorized that it might be necessary for maintaining a pregnancy.

In 1803, Nicholai Argunov created a painting of Praskovia Kovalyova (1768-1803), a talented opera singer from Russia who was born into a family of serfs. The portrait depicted her during her pregnancy, but sadly, she passed away just a few weeks after giving birth to her first child.

In 1900, a German researcher named Gustav Born made an interesting discovery. He noticed that monotremes, which are mammals that lay eggs and lack a placenta (such as the platypus found in Australia and New Guinea), did not have a corpus luteum in their ovaries. The corpus luteum is the remains of a follicle after ovulation, and Born believed it was needed for the development of the placenta. He also thought that the corpus luteum released a secretion that prepared the outer layer of the uterus for a fertilized egg to implant.

THE PREGNANCY HORMONE

After Born passed away, his student Ludwig Fraenkel continued his research. In 1903, Fraenkel found that removing the corpus luteum from pregnant rabbits caused them to have a miscarriage. Then in 1929, George Corner and Willard Allen discovered that they could prevent miscarriage in rabbits by giving them an extract from the corpus luteum. This extract was purified in 1933 and named progestin (also known as progesterone).

After an egg is released during ovulation, the corpus luteum starts to produce progesterone in anticipation of a potential pregnancy. This hormone helps to develop a thick lining of blood vessels in the uterus, which is necessary to support the growth of a fetus if fertilization occurs.

The corpus luteum continues to produce progesterone for around ten weeks, at which point the placenta takes over this role. During pregnancy, progesterone also helps to calm the uterus and prevent contractions that could lead to a miscarriage.

If the egg is not fertilized, the corpus luteum degenerates and stops producing progesterone. This signals the start of a new menstrual cycle.