Ben or bas nida - Rav Moshe Feinstein

 Igros Moshe (E.H. 4:14) Avoiding marrying a woman over concerns that she is a bas nida as her parents are not observant Jews, even though she is modest and a good person is G-d fearing and loves Torah,. The Rambam (Issuei biah 15) and Shulchan Aruch (E.H. 4:13) state that the child of a nida is blemished but is not a mamzer. All the commentaries state that the child is fit to marry even a cohen but the problem is he is blemished and therefore best to avoid.  Therefore the question is whether to avoid marrying them because of this blemish? In particular this woman even though she is modest and a good person and refined and apparently has overcome the spiritual damage - that perhaps her chidren  will be bad. However it is also possible that the mother was not actually a nida because she mght have gone swimming in a lake - and even a bathing suit does not prevent her from becoming purified. Furthermore our Sages (Kallah) Rambam and Shulchan Aruch described the blemish of ben nida as being bad personality traits such as brazzeness. And even though we have authorities that a mamzer is a brazen person, we don’t rely on that to say that if the person is not brazen  he can’t be a mamzer The Nachlas Yakov said that this just means if a person is brazen he should be suspected of being a mamzer and needs investigation. However since a ben nida is not prohibited to marry either by the Torah or rabbis it is possible to rely on the existence or absence of bad personality traits to determine whether she is a bas nida,and in this case with her good midos to assume she is not a bas nida and that the mother must have gone swinming in a body of water simiar to a mikve. Similary the son of sinners who is modest and refined can be assumed not to be a ben nida. .