In preparation, are parents permitted to buy clothes and other things for the baby that was not yet born? If not, why?

Halacha wise there is no problem in purchasing products for a baby who has not yet been born. If it is more convenient, one should certainly buy what is needed and trust that Be'ezrat Hashem all will be fine, the birth will be peaceful and in good time. There is no reason to fear from Ayin Hara. Rabbi Ro'i Margalit