Pregnancy blood and screening tests – are they necessary?

Prenatal testing: the basics

When you’re pregnant you'll be offered tests to:

• make your pregnancy safer 
• see how your baby is developing and how you’re doing 
• screen for medical conditions. 

(NHS Choices, 2017a)

It’s for you to choose whether you want to have any of the tests. But to make this choice, you’ll need to know exactly what they’re all testing for as well as their pros and cons.

Pregnancy BMI: weight and height check

You'll be asked to step on the weighing scales and have your height measured at your booking appointment. This allows your body mass index (BMI) to be calculated so you can receive recommendations for weight gain depending on your BMI at the beginning of your pregnancy (NHS Choices, 2017a).  After that, weigh-ins won’t be a regular thing.

Most women put on 10 to 12.5kg during their pregnancy, generally after 20 weeks. The majority of the extra weight is because the baby is growing, but your clever body also stores fat to make breast milk later down the line (NHS Choices, 2017b; PubMed Health, 2018).

Pregnancy urine samples

One thing you’ll do a lot of while you’re pregnant? Wee into a pot. You'll be asked to give a urine sample at your antenatal appointments so it can be checked for a few different things, including protein (NHS Choices, 2017a). 

If protein is found in your wee, it might mean you have an infection that needs to be treated. It could also be a sign of pre-eclampsia (NHS Choices, 2017a).

Blood pressure tests in pregnancy

You might feel lightheaded if you get up quickly in the middle of your pregnancy. This is because many women have lower blood pressure around this time. It’s not a problem but talk to your midwife if you're concerned. You’ll have your blood pressure checked every time you have an antenatal appointment in any case (NHS Choices, 2018a).

Around 10% to 15% of women have a rise in blood pressure later in pregnancy and this could be another sign of pre-eclampsia (NHS Choices, 2018a,b). Do let your midwife and doctor know straight away if you had high blood pressure before you got pregnant (American Pregnancy Association, 2015; NHS Choices, 2018a,b).

Blood tests in pregnancy

You'll be offered some blood tests during your antenatal care. Some of these blood tests are for screening for conditions like thalassaemia, sickle cell disease or chromosome abnormalities (NHS Choices, 2018c).

Screening tests can tell you whether your baby has a high or low chance of being born with them. If the screening test returns a high chance, you will be offered a diagnostic test, which can tell you for certain whether your baby has a particular condition. Screening is recommended for HIV, syphilis and hepatitis B when you’re around eight to 12 weeks pregnant (NHS Choices, 2017a).                

Blood group and rhesus status

There are a few reasons why knowing your blood group is useful. You might need to be given blood for conditions like heavy bleeding (haemorrhage) during pregnancy or while giving birth. 

Another reason to test your blood group is to know whether you’re rhesus negative or rhesus positive. This is because women who are rhesus negative are at risk of rhesus disease, which is when they develop antibodies that attack their baby's blood cells. Rhesus disease can cause anaemia and jaundice in the baby (NHS Choices, 2018d).

If you're rhesus negative, you can get injections during pregnancy to stop you from producing these antibodies. This is totally safe for you and the baby, of course.

Pregnancy and anaemia

You’ll be offered screening for anaemia (iron deficiency) at 28 weeks (NHS Choices, 2017a). It’s useful to know whether you’re anaemic because it causes exhaustion and will make things harder for you if you lose a lot of blood when you give birth. If you do have anaemia, you can take iron and folic acid for it.

Gestational diabetes and the oral glucose tolerance test

Gestational diabetes is a type of diabetes that happens during pregnancy. Gestational diabetes is more likely to appear during the second half of your pregnancy but it can crop up at any stage (NHS Choices, 2016). Any woman can develop gestational diabetes during pregnancy but you're more likely to get it if:

• your BMI is above 30
• your previous baby weighed 4.5kg or more when they were born 
• you developed gestational diabetes during a previous pregnancy 
• one of your parents or siblings has diabetes 
• your ethnicity is south Asian, Chinese, African-Caribbean or Middle Eastern.

(NHS Choices, 2017a)

If one or more of the risk factors above applies to you, an oral glucose tolerance test (OGTT) might be offered to you. It takes about two hours.

The oral glucose tolerance test involves a blood test in the morning when you've had nothing to eat or drink, then a glucose drink. After resting for two hours (take a good book), they’ll take another blood sample to see how your body is dealing with the glucose. This test is usually done when you're 24 to 28 weeks pregnant (NHS Choices, 2016). 

With all of these tests, it’s for you to choose whether you want to have them based on the information given to you and any conditions you might have. Talk through your options with your midwife or GP.

This page was last reviewed in March 2018.

Further information

NCT's helpline offers practical and emotional support in all areas of the antenatal stage, birth and early parenthood: 0300 330 0700.  We also offer antenatal courses which are a great way to find out more about birth, labour and life with a new baby.

NHS Choices gives full information on the checks and tests offered when you are pregnant.