Hi hun
I had thrush when i was pregnant and used the pessarys and the cream, the leaflet is saying be cautious to cover their backs, the pessary is only going int the vaginal wall, not through the cervix, so it won't be in the womb hun...
Being a female is a bitch, i get really itchty and thrush somethiem before AF is due!
Being underweight can have the same effect as being overweight, on the fact that you may not ovulate, but like you say you have been underweight all your life, so its not as if its new to your body, and as you say your mum and nad are skinny, so its in your genes, try to relax, thats worse for ovualation if you stress yourself out, take that from one who knows, i have been ttc for over three years now...
Its a problem if you all of a sudden loose LOADS of weight and become very underweight, or put on lots and lots of weight, then ovulation may be affected, but you seem healthy and have regular periods, so i would not worry, if you are worried then speak to your GP hun
Ps if i was you i would take the pessary asap, as that would do more harm than not taking it, good luck re ttc, and come and join us on the ttc thread xxxx