Not every "irrational" belief is destructive.
Some women who miscarry choose to believe (or their cultural traditions encourage them to believe) the desired child is going through a process of fetal death and re-conception which will make the child physically stronger and spiritually powerful.
In such a world-view the stress of multiple miscarriages is mitigated by the perception that the timing of the child entering the world just wasn't right and the child will arrive at the ripe time. Such a perception can lend itself to creating a relaxed mentality in both partners, controlling stress hormones and making successful gestation more probable.
Contrast this with a clinical perspective of each fetus being genetically unique and the person-hood (personality, intelligence, "spirit") being directly linked to the unique genetics and material components of each, separate human. [As opposed to the more realistic environmental & social factors which create epegenetic variations (and which may be consistent among siblings), the memetic milieu which shapes an individual's psychology, etc.]. Drug cocktails, fetal surgery and other advanced interventions are prescribed to avoid miscarriage...treatments most of humanity before us (and likely after us) could not begin to fathom.
The clinical approach often focuses on finding the cause of miscarriage and has value in its own right. But exclusively, because of its focus on identifying causation and predicting future events (& mitigating them), a woman can easily become stressed, anxious, isolated and self-doubting over a process that is extremely common to the human condition. Clinical science can adapt and become more humane, but until that time traditional folkways will maintain their role as a supportive therapy.
Another instance to consider: The Bari (a population in Venezuela) have held a traditional belief that a fetus develops not simply from the initial act of fertilization but through successive washes of semen. They seem to have some concept that the initial act has primary importance because they emphasize the importance of a wife having her husband as an exclusive partner for that time-frame, yet they encourage women (this practice is dwindling with western influence) to select a secondary father for the child--to take a lover during her pregnancy. This second man provides nutritional support for the woman through her pregnancy and for the child after it is born. The secondary man is viewed to be providing a service to the husband, who might otherwise waste away from trying to supply enough material to guarantee a successful pregnancy. This testifies to the ability of human culture to adapt to resource constrictions and use memetic engineering to mitigate the territorial concerns of our species which can impede our survival. Lest you view the Bari as an anomaly, variations of this belief have been observed in cultures from the Na of China to populations in New Guinea, India and other regions of South America.
From a purely "rational", "scientific" perspective (as such concepts stand today) these poor uneducated individuals in traditional societies don't know any better and need to be informed of modern medicine. However, their worldview is one which arose from environmental constraints and cultural adaptations which should not be dismissed purely as primitive superstitions. "Enlightening" them about scientific realities might first cause them to disparage traditional social structures and expose them to exploitation and destruction by other cultures. It may also rob Western science of the opportunity to reconsider its assumptions about biology.
We should not avoid such questions as: Does extra sexual activity during pregnancy have an impact on a woman's internal chemistry and thus the condition of the fetus? Does the introduction of another partner impact the immune system of the woman and/or fetus? (I'm not saying this to assert there must be a positive impact simply because this is a traditional folkway that has been preserved...I am merely saying it's time science evolved to thoroughly investigate traditional hypotheses and get down from its high horse. Even disproven theories benefit science.)
It is important that we value what we can learn from traditional beliefs just as much as we value what we can teach these cultures. We need to cultivate a symbiotic exchange over previous colonial approaches.