How can we know that medicines will not cause birth defects without testing them on animals?
The medical principle called Karnofsky's Law states that any substance can be teratogenic (cause birth defects) if given to the right species, at the right stage in development, and in the right dose. Common table salt or even water can be teratogenic to some species in certain circumstances! Therefore, science has already told us that any medication can cause birth defects in some creatures. 
In addition, agents that are teratogenic to some species may have little or no teratogenic affect in others. Of 1,200 chemicals that caused birth defects in animals, only 30 were proven to affect humans.  As the book, Chemically Induced Birth Defects records:
"In approximately 10 strains of rats, 15 strains of mice, 11 breeds of rabbits, 2 breeds of dogs, 3 strains of hamsters, 8 species of primates and in other such varied species as cats, armadillos, guinea pigs, swine and ferrets in which thalidomide has been tested, teratogenic effects have been induced only occasionally."
Rats, popular lab animals, have been shown to get birth defects from almost every chemical that causes birth defects in humans. But they also get birth defects from hundreds of drugs that are safely used by humans! If chemicals that harm rat offspring do not cause birth defects in humans, the animal tests are meaningless and non-predictive.
So what is teratogenicity-testing in animals good for and why does it continue? As obstetrics professor Dr. D. F. Hawkins points out:
"The great majority of perinatel toxicological studies seems to be intended to convey medical and legal protection to the pharmaceutical houses and political protection to the official regulatory bodies, rather than produce information that might be of value in human therapeutics."
As Karnofsky's Law postulates, researchers can eventually inflict birth defects on a species with substances that are teratogenic in humans. But to what purpose? Non-predictive animal experiments are of no human value. They only deplete valuable research funding that might otherwise be of true medical value.