Clawed Frogs: Nature’s Pregnancy Test

by pregnancy journalist

Mating and breeding are fundamental aspects of surviving as a species and although many animals have breeding seasons, humans as a species, do not. This means that women don’t have specific times during the year in which they are more fertile and willing to engage in mating in order to get pregnant. Instead, we go through monthly fertile cycles between the first menstruation (menarche) and menopause. This is good on one hand because it is not necessary to wait for a specific month or months of the year in order to get pregnant but also bad because, as it can occur at any given time, women might fail to notice they are pregnant and not take basic pre-natal care to ensure the survival and best development of the baby.

Old school pregnancy tests

For many reasons it quickly became important to find out if a woman was pregnant or not as soon as possible. Was the queen finally going to provide an heir? Was the future virgin bride not so virgin after all? Was the husband that had just come back from the sea really the father? Being able to tell, with a certain accuracy, if a woman was pregnant or not would solve many problems but we still have no idea how our (very ancient) ancestors were doing it (if they were at all) since no information was ever found. We are aware however that the first known pregnancy test started to be used in the ancient Egyptian and Greek cultures with the use of urine and grains. They would introduce, on one side, urine of the lady that wanted to find out if she was pregnant, and on the other side (as the control subject) the urine of a priest, on bags of barley and wheat. Both bags would be observed for how long germination would occur. If a lady was with child her grains would sprout a lot faster than the priest (we could easily infer he would not be pregnant).

Throughout history many other “urine analysis” techniques were used to find out if women were pregnant, some of them more scientific than others; but it wasn’t until the last century that animals started to be used in the procedure.

With the discovery that the hCG (human chorionic gonadotropin) hormone was produced by the placenta during the first trimester of pregnancy, scientists were able to design a better and more reliable way to test for pregnancy.

This discovery, in the 1930’s, boosted the development of pregnancy test worldwide with very big implications. For the first time in known history, scientists were using live animals such as mice and even rabbits for their testing. Infantile females (non-sexually mature) were injected with the urine of the woman to be tested and later on killed and dissected in order to look for the presence of ovulation in the animal. The ovulation would have been triggered by the hCG presence in the injected urine of pregnant women and would pose as a positive result.

This method, however, meant that for every tested woman we would end up with a dead mouse or rabbit. Luckily for humanity shortly after, still in the 1930’s, a gentleman named Lancelot Hogben found out that he could use a specific genus of frogs, the Xenopus, to get the same results but without having to kill the animal.

Xenopus: 1930’s-1950’s pregnancy test

Clawed frogs (Xenopus) were subjected to the same method; the urine of the woman was injected in its dorsal lymph sac and if the frog produced eggs in the first 12 to 24 hours the woman was with child. The big difference between mice and rabbits and these frog species is that amphibians, unlike mammals, have external fertilization so the new eggs could be easily observed without having to kill and dissect the animal.

By the 1940’s, this test, named the “Hogben Test” in honour of its discoverer, was already widely used in hospitals.

Many facts helped this method achieve worldwide recognition:

  • It was extremely accurate
  • It was very easy to perform
  • Results were achieved in 12-24 hours
  • The frogs were easily bred
  • The frogs were conveniently kept in aquariums
  • Amphibians in general have large eggs which can be easily examined and manipulated
  • Eggs are released outside the animal (they use external fertilization)
  • Clawed frogs reacted to the hCG hormone released by pregnant women
  • Since the frogs didn’t have to be killed and dissected in order to have their eggs examined, they could be used multiple times
Clawed Frogs: Nature’s Pregnancy Test

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