Progesterone: The Unsung Hero of Pregnancy

In the 1920s, scientists made a major breakthrough by identifying estrogen and linking it to female reproductive health. At the time, many believed that estrogen was the only hormone driving female fertility. But not all researchers agreed. Years earlier, in 1897, Scottish embryologist John Beard had proposed a bold idea: the corpus luteum—a temporary endocrine structure formed after ovulation—might play a critical role in sustaining pregnancy.

A Closer Look at the Corpus Luteum

In 1900, German scientist Gustav Born added more weight to Beard’s theory. He observed that monotremes—egg-laying mammals like the platypus—lacked a corpus luteum and did not develop a placenta. Born hypothesized that the corpus luteum was essential for placental development and believed it released a substance that prepared the uterus for embryo implantation. His insight laid the foundation for what would later be recognized as one of the most important reproductive hormones.

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The Discovery of the “Pregnancy Hormone”

After Born’s death, his student Ludwig Fraenkel carried the research forward. In 1903, Fraenkel discovered that removing the corpus luteum from pregnant rabbits led to miscarriage, highlighting its vital role in early pregnancy. A few decades later, in 1929, American scientists George Corner and Willard Allen made a significant breakthrough. They successfully prevented miscarriage in rabbits using an extract from the corpus luteum. By 1933, they had isolated and named the active compound: progesterone.

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The Role of Progesterone in Pregnancy

Progesterone is often referred to as the pregnancy hormone for a reason. After ovulation, the corpus luteum begins producing progesterone, preparing the uterus for a possible pregnancy. This hormone helps thicken the uterine lining, enriching it with blood vessels to nourish a fertilized egg.

If fertilization occurs, the corpus luteum continues to release progesterone for about 10 weeks, ensuring the uterus remains calm and stable. It helps prevent uterine contractions that could trigger a miscarriage. After this period, the placenta takes over progesterone production, maintaining hormonal support throughout the rest of the pregnancy.

However, if fertilization does not happen, the corpus luteum gradually breaks down. As progesterone levels fall, the body recognizes that pregnancy did not occur, prompting the start of a new menstrual cycle.

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Key Takeaways for Curious Minds

• Progesterone is essential for maintaining a pregnancy, especially in its early stages.
• The corpus luteum, once thought to be insignificant, is now known to be a key temporary hormone-producing structure.
• The discovery of progesterone helped revolutionize reproductive medicine and is foundational to many fertility treatments today.
• Without adequate progesterone, the uterine environment cannot support embryo implantation or early fetal development.
• Understanding the balance between estrogen and progesterone has shaped our modern knowledge of the menstrual cycle, conception, and pregnancy care.

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.