Problem solved

I'm 36 and had my first child a year ago. When I was pregnant, I found out that my blood type is B+. I don't know why, but it kept niggling at me and I asked my parents (long-divorced and not in contact with each other) what their blood types are.Both are blood donors so they know for sure, and it turns out they are both type O. Mum O+, Dad O-.

I'm pretty sure that two type O parents can't have a type B child. Is this correct? I have one sister, and she's also type O. I'm thinking that either my dad is not my biological father, or there was a hospital mix-up and neither of my parents are my parents. It's most likely to be the former, I imagine, but I feel the need to find out for certain – my dad is my dad, we're very close and I would never, never want him to know any of this if it is indeed the case.

I want to ask my mum, but I'm afraid to do that in case she's not my mum either, because if that's the truth I wouldn't want it to come out at all.

Is it possible to get a DNA test done to at least establish whether my mum is my mother, without her knowing about it?

Anon, via email

You cannot get a DNA test done on someone without their consent. It is illegal otherwise, not to mention immoral. The first thing I'd say to you is to check and then double check the facts.

I spoke to Dr Roderick Neilson, a consultant haematologist in the NHS. He said: "It is very important to ensure the blood groups you and your parents have been told you are, are completely correct, as errors do happen and it could be that one or all of you are not the blood group you think."

Neilson went on to say that in a recent study of British military personnel, approximately 20% of the blood groups recorded on their ID cards were incorrect, "to the extent that blood groups are now no longer recorded on [military] ID cards". Modern practice is to recheck the blood group of a person every time they are given blood for whatever clinical reason; every bag of blood donated also has its blood group checked on donation.

Neilson explained that there have been nearly 300 blood groups identified. From a clinical point of view "the most important ones are the ABO and Rh – previously know as Rhesus – blood group systems."

Within ABO there are four main blood groups: A, B, AB, and O.

So, the big answer to your question: if (and I want to stress again that you must ascertain the facts) you find you are definitely ABO blood group B and your parents are definitely group O, then you are correct, you cannot be the child of both your parents. (You cannot have recessive genes with the ABO blood group, but you could with Rh blood groups.)

If I were you, the first thing I would do is get your own blood group retested. Ask your GP to do it and tell him why. You may well come back as ABO group O in which case you can relax. If you're definitely group B, then the next stage is to ask your parents to have their blood retested. Obviously this isn't so straightforward and only you can decide if you want to do this – and tell them why.

Is this something that was niggling away at you before you knew your blood group? If I found out that, according to their blood groups, my parents couldn't possibly be my parents, I would doubt the results, not my parentage. Certainly having a baby can prompt one to look again at family matters and probe deeper into things. But I wonder if you've had doubts for a while and if so, why?

I can't say what your situation is, but – if all the blood groups are as you say – then there are various possibilities (a hospital mix-up is one of them, but extremely unlikely). And one of them is that your father isn't your biological parent.

So your mother might be a good place to start. She may be waiting for an opportunity to talk to you about this.

But first check as many of the facts as you can. If your blood groups are all as you think, feel free to write back to me about what to do next and I can go into things in more detail.

Your problems solved

Contact Annalisa Barbieri, The Guardian, Kings Place, 90 York Way, London N1 9GU or emailannalisa.barbieri@mac.com. Annalisa regrets she cannot enter into personal correspondence.

Follow Annalisa on Twitter @AnnalisaB

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