The reasoning can be SO varried...
And, unfortunately (or not depending on your outlook) most women having unprotected sex get 'pregnant' between 4-6 times a year (massive genetic screwups, usually in the DNA zippering incorrectly... these are NOT savable situations, no matter how much one might like; we're talking like acid for blood, or no bones, or 12 arms and no body or internal organs. NOT birth defects, but lethal DNA zippering issues) means that a woman is either not late at all, or is just a week or two late in their periods.
Which is the foundational reason why most OBs won't even start testing for problems until you've had at least 3 'real' miscarriages in a row. (meaning 6-8 weeks pregnant at a minimum). Which can seem VERY unfair, if you tested positive at 2 weeks, but it was a chemical pregnancy.
((I do NOT know your background, they could all have been at 3mo, 6mo, just doing due dilligence background since I've been through this a *bunch*))
With me'self, I lost my daughter between months 6 & 7 (placental abruption), but it wasn't until *several* misses later that we found out that the problem for ME is twofold:
1) I grow my placentas too big. With very little provocation, they rip away from the wall, killing the baby (and me if I'm not close to a hospital to stop the hemorrhaging)
2) Pregnancy hormones trigger cancer in me (which creates a not so great "host" situation. The faster the cancer grows, the faster the placenta grows in my case).
With this information firmly in hand I had some of the most BORING / FRUSTRATING 9mo of my life. Modified bedrest the entire time, and multipe surgeries to try and keep the cancer in check (5? I think by the time I delivered? It's been awhile. I had several more after my son's birth).
There is NOTHING to test for with my problem... it was a combined looking at the problem and figuring out commonalities.
With other women... there are literally HUNDREDS of causes (excluding badly zippered DNA, which happens all the time in every other cell as well, our body is designed for handling badly zippered DNA, it's when it doesn't -and the mutated cell AND it's neighbors don't self destruct- that we get cancer).
As mentioned; these cases are *usually* handled by a fertility specialist. And they need data to work with from past miscarriages. Blood work, lab work, tissue samples, imaging, etc.
It can be as simple as a placenta that grows too large, or a nutritional deficiency or as complex as increased stress levels (when the mum's body is under extreme stress -including ALL sorts of stress, from 'classic' to dieting, the body usually aborts the fetus, regardless of what stage it's in, in an attempt to save itself. The body is going off of chemical messengers... it doesn't know if the cortisol and adrenaline are coming from a terrible job or being in a war zone; dieting or famine). Hundreds, and hundreds of causes.
My condolences on both your loss and your frustration,
R