welcome to HealthcareMagic
You are right in your fear regarding pregnancy
there are two aspects to keep in mind :-
-pregnancy at or above 40 yrs has more risk during delivery as well as there are high chances for the baby to have congenital malformations or birth defects.
-the problems you are having like thyroid; osteoporosis
and the disc prolapse
and even the weight.
If you have 2 children you should have avoided getting pregnant anyway it will be better for you to consult a Gynaec and your Orthopaedic doctor for opinion.Consult and discuss with both doctors and then decide.To abort or not then you will have to decide.