This seems the best topic for your question, Beattie. Pregnancy and motherhood is enough to make anyone insecure: hormones or not, donor or not. We just have an extra topic from the selection of things we could worry about :-).
My DD is only 1.5, but my husband mentions it every now and then to her, just so she will always have known, and to get used to it and so as to avoid having "The Big Telling" at some point in future.
We live abroad, and telling was always the way to go for us. So much that we decided to pay for our own treatment so our children could contact the donor at 18. Where we live I could have had a large part of the treatment reimbursed, but we would have to have had a life-long anonymous donor (current law).
I read somewhere about explaining the difference between secrets and privacy, and think we would try that once she can understand the concept. Before that there is not much point, but the topic of how she came to be will probably not be discussed with outsiders much. We do not make it a big secret, but of course do not just blurt it out any time. However when we do tell people they actually have a hard time believing it and think we are joking... They would never understand or believe what a small child is telling them on the subject, or just think she does not grasp the details if where babies come from.
My husband is a vet, which means most of the village know who he is, illustrated by the fact that a client of his mentioned she heard we were going to move, and knew exactly which house we have just bought... It makes him slightly afraid of the moment the news of his infertility spreads beyond close aquaintances and trusted friends, but that hurdle will just have to be taken when we get to it.
