When should I tell them I'm pregnant?

First off if you request an ultra sound then your midwife cannot say no, that is your decision. Secondly if you have such health problems you should not be going to a midwife - you should go to a doctor who can more closely moniter you. At 14 weeks it is probably safe to tell the family although something can always happen. Stressing about it will be more of a problem to the health of your baby than anything else. Studies prove that stress can be damaging to the unborn baby. I would just tell them and get it out in the open then you don't have stress about it.