There is nothing unique to cannabis that makes it a psychologically addictive than any pleasurable activity, like I mentioned earlier, such as sex, video games, or watching t.v. Many of the studies showing negative effects are of questionable validity due to the government sponsored war on drugs. Furthermore, most of these studies contradict each other, concluding possible outcomes that are not peer reviewed to be correct, unlike say alcohol.
There are cultures in the world in which MJ is socially accepted with no exception to use during pregnancy and these cultures, such as Jamaica, do not show generations of birth defects or psychological issues. Birth defects from harder drugs are documented because upon scientific study they become obvious. There is quite the difference between pain medication and heroin, and many women would take pain medication, as prescribed, during pregnancy because it's socially acceptable.
You also don't need refined sugar, toxic make-up, toxic air, the effects of electromagnetic radiation, cows milk, and many other substances that are proven to harm a developing baby. Yet you don't hear people telling women they shouldn't risk having a chocolate bar or using cosmetics.
So not only is there no concrete proof of negative effects of MJ use during pregnancy, there are documented benefits to use for some mothers, such as those with severe morning sickness or eating disorders, or even anxiety issues. These babies are much better off being exposed to MJ than not getting enough nutrients, which is the biggest source of complications at birth.