I asked my gyn this question and he said no, you can't tell from looking at menstrual flow. Mine has been scanty these past few months (2 days), but when I went to the doc he did an ultrasound which showed that my lining was fine (11 mm). However, I've been taking baby aspirin and Red Raspberry Leaf to help build it up as well.
I wondered this as af doesn't last long (one day heavy, three days barely noticeable).
I asked my doctor about it and she didn't think that was too much of a problem. She advised eating healthily and taking conception aid, like seven seas or fertilaid.
I also previously took a herbal remedy to help with pms - Agnus Castis. I got it in the health food shop and it seemed to really help regulate things. I'm not sure whether you can take this when you're ttc but it might be worth reading up on.
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