The one that is decided within hours

One practical question comes up faster than any of the others and almost nobody warns a woman about it.

After a miscarriage, is the bleeding nifas, which suspends prayer, or is it istihada, which does not. The answer turns on the same gestational question of whether the child had discernible human form, and it therefore differs by case and by school.

There is one number that makes the question askable rather than vague. Islamqa, in fatwa 45564, holds that the test is discernible human form rather than a date on a calendar, and puts the earliest point at which features may appear at eighty one days from conception, on the authority of the Standing Committee, Ibn Baz and Ibn Uthaymin. That is one school's answer and it is not the rule. Other schools frame the threshold differently, and the same site is not consistent with itself, because its fatwa 50106 says flatly that bleeding after a four month miscarriage is nifas. Take the number as something to ask with. Do not take it as something to decide with.

This matters within hours rather than weeks, because she is lying there deciding whether to pray, on a day when getting it wrong in either direction will sit on her for months.

Ask, quickly, and ask a person rather than a search engine. One message to a scholar or to a knowledgeable woman at your masjid, with the gestation in it, gets a real answer the same day. If you cannot get an answer immediately, that delay is not a sin either.