Looking at the Gemoro, people used to believe that babies born at 7 months were more likely to survive at 7 months than 8 months. A male is born face down whereas a female is born face up. Another source describes 2 channels in the male sex organ; one for urine and one for semen? How did these beliefs arise and how could people have believed things like this if they are demonstrably false? There are also various tereifos that were fatal, but no longer. Was it really the case that no one bothered to check?Alternatively, has nature changed? I believe these ideas are myths which is why I've posted them here. Rambam speaks about astronomy. In those days they couldn't prove things one way or another. The examples above would have been testable in those days as they are today. Is it possible to test if the Talmud's description of human anatomy was correct eg finding a skeleton over 1800 years old. Another example is the idea that people have 248 bones and 365 tendons or muscles. Nowadays we know an adult has 206 bones. Perhaps they looked at a child's body or perhaps they followed an accepted tradition which had never been checked. In summary: Can the scientific beliefs in the Talmud be tested? If reality was the same as today, why did no one point out the errors?
When examining Maimonides' medical writings and comparing them to the medical advice of the Talmud, one is often left in shock. Many, if not all of the ideas contained in the Talmud are erroneous when in comparison to the findings of modern science. How can such pious talmudic rabbis err? Is it possible that such holy men could make mistakes? Maimonides explained that the ancient rabbis were not experts in science and often had to work with the science of their day, and since the science was frequently wrong, it is no surprise that they were also in error.
Thus, the ancient rabbis had limitations and did not always understand science fully. As a result, they are frequently wrong because they relied on the primitive science of their age. But since science progresses, it is no surprise that Maimonides even admits that future generations will understand science better than he does.
Did people’s nature change? No. Natural law is fixed and needs no change. G-d created the world perfect, as the Bible says "Very good."
 (Guide to the Perplexed, III:14)
 See Guide 2:24
With these types of questions, one needs to be very careful about how he understands the words of the Sages. Often the contradiction comes from a mistaken or simplistic understanding of their words, and a deeper understanding shows that there is no contradiction.
For example, the first part of the question, about the seven month and eight month babies. Chazal say that a seven month baby can live, and an eight month baby cannot, unless the eight month baby is really a seven month baby who was born late. This is true; there are seven month babies who are perfectly healthy (one was born to a friend of mine), and an eight month pre-term baby cannot survive without medical intervention. Is a baby born at eight months more likely to be pre-term? Halacha seems to say yes. Compared to a seven month baby? Is that an easy comparison to make?
Skipping to the case of trefos, Chazal give us a list of trefos, and there is a dispute if an animal with such an injury will necessarily die. We see that, for some of these injuries, the animals live. That could mean that the reality is like the one side of the dispute, or it could mean that these injuries are not always fatal, and just that such an injury could cause death. The other side of the dispute could hold that some trefa injuries are not fatal at all. Or it could mean that we don't know the proper definition of those injuries that Chazal listed.
With regard to things discussed in the Gemorah, they were frequently very easily observable and determinable even in ancient times. They couldn't have simply gotten those things wrong.
(The answer which is usually given about the Gemorah which mentions people having 248 bones is not that nature changes. That Gemorah also explicitly says they dissected corpses and came back to ask their Rebbe that they didn't find 248 bones)
Even without the Gemorah, based on non-Jewish medical history books I see no question that things have changed since ancient times. It's simply inconceivable that in ancient times they practiced all those wacky medical techniques (not mentioned in the Gemorah)if they helped no one. They had to have helped back then. If they were really so ineffective people would have noticed. They never could have caught on and been practiced to such a large extent if people didn't see any improvement that they could clearly attribute to having those medical techniques done.