How accurate are gender blood tests?

It is the question we are asked most often, and the honest answer has two halves: the test itself is extremely reliable, and the sample is where accuracy is won or lost.

What the laboratory is actually looking for

During pregnancy, small fragments of your baby's DNA circulate in your bloodstream. The laboratory looks for Y-chromosome material. If it finds it, you are expecting a boy. If it does not, you are expecting a girl. There is no estimating or guessing involved — it is a direct look at genetic material.

The accuracy figure, and the condition attached to it

Laboratories quote accuracy of over 99% from 10 weeks of pregnancy. That figure holds when the sample is a proper venous draw taken at the right stage of pregnancy. It does not describe every sample that arrives at a laboratory.

The three things that cause a wrong result

1. Testing too early

Before 10 weeks there is often too little foetal DNA in your blood for the test to find Y-chromosome material even when it is there. That produces a false girl result. This is the single most common reason a gender blood test is wrong.

2. Not enough sample

Finger-prick collections frequently produce too little blood, or blood that has begun to clot on the collection card. The laboratory either reports it as inconclusive or works with a marginal sample.

3. Contamination

This one surprises people. Male DNA in the room — a partner helping with the kit, a young son nearby — can transfer onto the collection card and produce a boy result for a girl. It is a genuine, documented cause of error with home kits, and it cannot happen in a clinic setting where the sample goes straight into a sealed tube.

Almost every cause of an incorrect gender blood test result comes down to how and when the sample was taken — not the laboratory analysis.

What about twins?

The test still works, but it can only tell you so much. If male DNA is found, at least one baby is a boy — it cannot say whether both are. If no male DNA is found, both are girls.

How to give yourself the best chance of a reliable result

• Wait until you are at least 10 weeks pregnant.
• Have the sample taken from your arm by a phlebotomist, not from your fingertip at home.
• Use a provider whose price includes the draw, so there is no incentive to cut corners on collection.
• Ask what happens if the result is inconclusive before you pay.