Rhesus D

There's a good chance that before you became pregnant, you might not even have known which blood group you belonged to. It's probably something that didn't seem important, or something you might have been curious about, but never saw a need to investigate further.

But once you do conceive, it's vitally important that your blood group and your rhesus status are ascertained and this is usually done at your first antenatal check up or booking visit. A blood test will be carried out and amongst other things, it will show your blood group (either A, B, AB or O) and your rhesus status (which will either be positive or negative).

If you are rhesus positive (RhD positive) you will have a protein on the surface of your red blood cells called D antigen. But if you're rhesus negative (RhD negative) you won't have this. It's much more common for people to be RhD positive, although there are some variations between ethnic groups. Your chances of being RhD positive are 85% if you're of white European origin, 90% if you're of Asian origin and 94% if you're of African origin.

So why does my rhesus status matter?

You're probably wondering exactly why your rhesus status matters. In fact it's only of importance if you're a mum who's RhD negative and you're carrying a baby who's RhD positive. This is entirely possible, as babies can inherit the RhD positive status from their father.

Why would these differing RhD statuses matter? Well, during pregnancy and during the birth itself, if any of your baby's blood gets into your own bloodstream, there could be a reaction in your immune system to your baby's D antigen. Your body would treat the D antigen as though it were a foreign invader and start producing antibodies to fight against it in a process known as sensitising.

It sounds dramatic, but there is usually nothing to worry about in your first pregnancy when sensitising is rarely harmful. It's only when, in the future, you get pregnant for a second time, with a baby who is RhD positive that it becomes a problem. If this happens and no preventative measures are taken, the antibodies produced by your immune system may cross the placenta and attack your baby's blood cells. There could be serious consequences for your baby such as jaundice or anaemia, even heart or liver failure, in a condition known as haemolytic disease of the newborn (HDN). This is why it's important for medical staff to clarify your RhD status, because there are very effective steps they will take to prevent any problems.

How does my baby's blood enter my bloodstream?

Your baby's blood may enter your bloodstream in a number of different ways. It could happen if you have an ectopic pregnancy, a termination, or a miscarriage or vaginal bleeding after 12 weeks. Also invasive tests such as amniocentesis or chorionic villus sampling may cause it, as well as receiving a hard blow to your stomach and having external cephalic version (a procedure performed to turn breech babies head down).

During birth there is a high risk that your blood and your baby's will mix, especially if you have a caesarean section.

Is there anything I can do to prevent problems?

As an RhD negative mother, once your immune system has produced antibodies they will always be in your blood. So, it is extremely important to prevent them being made. Luckily this is a very easy thing to do, with a simple intra-muscular injection of anti-D immunoglobulin. The anti-D gets to work quickly and destroys any blood cells from your baby that have got into your circulation, before your immune system kicks in to make antibodies. In this way, you won't have any antibodies to cause problems during this pregnancy or your next.

Anti-D is made from donated plasma, which is the clear yellow coloured part of blood. It is carefully screened for HIV, hepatitis B and C and variant CJD. It's been used since 1969 and its widespread use has meant that HDN is now rare.

It is possible that you might have an allergic reaction to anti-D although it's unlikely and it won't harm your baby.

During your pregnancy your blood will be checked for antibodies both early on and at 28 weeks. It's very unlikely that you will have any as since anti-D was introduced incidences of women developing antibodies have declined dramatically. But if you are found to have antibodies, your midwife will refer you to a specialist in foetal medicine and your pregnancy will be closely monitored for signs of foetal anaemia. You won't be given anti-D if you already have antibodies, as it cannot remove them once they've been made.

If you're an RhD negative mother you should be routinely offered anti-D during your pregnancy at both 28 and 34 weeks of your pregnancy following the recommendation of the National Institute for Health and Clinical Excellence (NICE). You need to receive an injection of anti-D more than once because it can only remain within your system for around six weeks.

After the birth

Then once your baby has been born, a sample of blood from the umbilical cord will be taken to determine his or her blood group and rhesus status. If your baby is RhD positive, you will receive a further anti-D injection to stop your immune system from producing antibodies. It's important that this is done with 72 hours of the birth.

Just after you've given birth, a sample of your blood will be taken to see whether you have produced any antibodies. If this is found to be the case, you may be given a larger amount of anti-D.

If your baby is found to have an RhD negative status, the same as you, you won't need any anti-D because your immune system won't have been triggered to produce antibodies.

So, although some of the facts surrounding RhD negative status may appear worrying at first sight, it's important not to panic. Providing your RhD negative status is detected early on, there's no reason why, with anti-D injections, your pregnancy shouldn't proceed healthily and happily.