How does eating nuts when pregnant give my baby a nut allergy?

It doesn't. Who told you that one, that's funny!! I take it your prego, congrats!! Listen to what everybody says, but only believe less than half. chill out and enjoy you pregnancy peacefully!! Peace out!!

I ate nuts whilst pregnant neiher of my kids has an allergy

Allergies are caused by specific expressions of genes; you cannot cause your child to have a nut allergy by eating nuts while pregnant.

it don't, i was told not to have them for 3 months while i was having cravings for them and after 3 months they said i could eat nuts after all but i think it's because when you or someone in your family have a food allergy it could past it on to your baby.

Severe allergic reactions (e.g. anaphylactic shock) occur when the body's immune system reacts to a particular allergen or irritant that can be triggered by certain foods. The most common food products that cause reactions are peanuts, tree nuts, sesame, soy, fish, wheat, eggs, milk and seafood which if eaten in large quantities while pregnant or breast feeding, could cause the baby to develop an allergy to them because of the immaturity of their immune systems. This is also particularly important if your family has a history of allergy or asthma (asthmatics are highly susceptible to allergic reactions because of their sensitivity to irritants). Mothers of young childeren under 2 years of age are also advised to avoid use of the above foods because of their immature immune systems.