Conception and genetics

Your baby's characteristics will come from genes inherited from both parents.

Most people look like their natural parents in some way. As well as appearance, medical conditions, such as short-sightedness, asthma or hayfever, may run in a family.

Every cell in our body (with the exception of the sperm and ovum) contains 46 chromosomes. These are grouped in pairs, making 23 pairs.

The chromosomes act like a library of information. They are made up of long threads of a chemical that has become known as DNA (deoxyribonucleic acid).

Each thread has segments called genes. It is thought that there are around 50,000 genes. These genes determine physical characteristics such as skin colour, height and face shape, as well as perhaps having some influence on our personalities, intelligence and physical talents.

Genes also are linked to our health. It is becoming clear that faulty genes can cause or pre-dispose us to diseases.

Your baby's genes

Your baby will get half of his chromosomes and genetic information from you and the other half from your partner. Although our human cells have 46 chromosomes, the sperm and the egg only have 23 each.

The make-up of the 23 chromosomes in the man's sperm and the woman's egg is a random selection, which is why children of the same parents are not all the same.

At the moment of conception the sperm fuses with the egg and the chromosomes pair off. The genetic lottery has then begun!

Everyone, with the exception of identical twins, has their own DNA make-up. Identical twins have the same DNA make-up because shortly after conception, the fertilised egg split into two.

Why things go wrong

Like most processes, conception is not problem-free. Sometimes chromosomes are damaged or the wrong numbers are produced.

The risk of this increases with age as the genetic information in a woman's eggs ages with her. When this happens, a woman may have a miscarriage, or a child may be born with some kind of problem.

This is one reason why many fertility experts are keen for women who are thinking about starting a family to begin sooner rather than later.

As well as damaged or extra chromosomes, some genes that we inherit are faulty. In many cases, this has no effect on us at all as the genes are 'switched off'. Unfortunately, combined with our partner's genetic information, they can sometimes be switched back on again.

This is why certain conditions such as colour blindness, cystic fibrosis and sickle cell anaemia can jump generations. If all this sounds alarming, it's worth remembering that nature tends to get it right far more times than not, so genetic conditions are reasonably rare.

Genetic counselling

For couples worried about passing on a life threatening disease or serious birth defect, it is now possible to find out the actual risks by going to genetic counselling.

Genetic counselling:

  • evaluates your family and medical histories.
  • helps you to understand the risk of having a baby with a serious illness.
  • provides information on the risks to help you decide what to do.

Genetic counselling is carried out by specialist counsellors who have a nursing or medical background. It’s a very sensitive area so counsellors are chosen very carefully.

Couples may be asked about their family history as well as for a blood test or a swab. This information allows the counsellor to calculate the risk of the disease being passed on to a child. 

Alongside this information, couples discuss their own feelings about the risk and the possible options.

Who needs genetic counselling?

Overall very few couples will need genetic counselling, but it is advisable to talk to your doctor before you get pregnant if you have any concerns, especially if you can answer yes to one or more of the following questions.

  • Do either of your parents or close relatives have an inherited disease or birth defect?
  • Have either of you already had a child with a birth defect or genetic disorder?
  • Have you had three or more miscarriages?
  • Are you closely related, for example, are you first cousins or do you have a closely related family tree?
Read more about genetic issues

Read our top tips for getting ready to conceive.


  1. Macdonald S, Magill-Cuerden J (2012) Mayes’ midwifery, 14th edition, London Balliere Tindall
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Last reviewed on June 13th, 2017. Next review date June 13th, 2020.

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