Hi there! To answer your question, there exists the same question in the internet and the answer is thousands of experiments have been conducted worldwide to test Darwin’s initial hypothesis regarding natural selection. Almost all scientific findings today have been attributed to Darwin’s theory, thus they needed to conduct the scientific method to be able to come up with new learnings.
Oxytocin causes contractions during pregnancy, it also is the reason why mothers feel "motherly" towards their children after childbirth as there are high levels on oxytocin in the mother's blood.
Check the pic