Blood Tests During Pregnancy: Trimester-by-Trimester Guide

Overview of Pregnancy Blood Tests

Blood tests are a routine and important part of antenatal care. They help your midwife and doctors detect potential complications early, confirm your blood type, screen for infections, and monitor your health throughout pregnancy. While the number of blood tests may feel overwhelming, each one serves a specific purpose in keeping you and your baby safe.

This guide walks through the key blood tests you can expect during each trimester of pregnancy, based on current NHS and NICE guidelines.

First Trimester: Booking Bloods (8 to 12 Weeks)

At your first midwife appointment (the booking appointment), usually between 8 and 12 weeks, several blood tests will be requested. These are often called "booking bloods" and include:

• Full blood count (FBC) — Checks haemoglobin levels to detect anaemia, which is common in pregnancy. Also checks white blood cells and platelets. Your haemoglobin should ideally be at least 110 g/L in the first trimester.
• Blood group and Rhesus (Rh) status — Determines your blood type (A, B, AB, or O) and whether you are Rh-positive or Rh-negative. If you are Rh-negative, you may need anti-D injections during pregnancy to prevent your immune system from attacking the baby's red blood cells.
• Red cell antibodies — Screens for antibodies that could cross the placenta and affect the baby's blood cells. If detected, additional monitoring will be arranged.
• Hepatitis B surface antigen — Hepatitis B can be passed from mother to baby during birth. If positive, the baby will be vaccinated at birth to prevent infection.
• HIV test — Early detection allows treatment that dramatically reduces the risk of transmission to the baby (from approximately 25% to less than 1%).
• Syphilis screening — Syphilis during pregnancy can cause serious complications including stillbirth, but is treatable with antibiotics.
• Rubella immunity — Checks whether you have antibodies to rubella (German measles). Rubella infection in early pregnancy can cause congenital rubella syndrome. If you are not immune, you cannot be vaccinated during pregnancy but will be offered the MMR vaccine after delivery.

Additional tests at booking may include haemoglobin electrophoresis to screen for sickle cell disease and thalassaemia, depending on your ethnicity and family history.

First Trimester: NIPT (From 10 Weeks)

Non-invasive prenatal testing (NIPT) is a blood test that analyses fragments of the baby's DNA circulating in your blood. It screens for chromosomal conditions including:

• Trisomy 21 (Down's syndrome) — Detection rate greater than 99%.
• Trisomy 18 (Edwards' syndrome) — Detection rate greater than 97%.
• Trisomy 13 (Patau's syndrome) — Detection rate greater than 99%.

NIPT is a screening test, not a diagnostic test. A "high chance" result means further investigation (amniocentesis or chorionic villus sampling) is recommended, while a "low chance" result is very reassuring. The NHS in England offers NIPT as an evaluative rollout, and it is widely available privately. It can be performed from 10 weeks of pregnancy onwards.

Second Trimester: Glucose Tolerance Test (24 to 28 Weeks)

Gestational diabetes affects approximately 5% of pregnancies in the UK. If you have risk factors, you will be offered an oral glucose tolerance test (OGTT) between 24 and 28 weeks. Risk factors include:

• BMI above 30 kg/m².
• Previous gestational diabetes.
• Family history of diabetes (first-degree relative).
• South Asian, Black, or Middle Eastern ethnicity.
• Previous baby weighing 4.5 kg or more.

The test involves an overnight fast, a fasting blood glucose measurement, then drinking a 75g glucose solution. A second blood sample is taken 2 hours later. According to NICE guidelines, gestational diabetes is diagnosed if the fasting glucose is 5.6 mmol/L or above, or the 2-hour glucose is 7.8 mmol/L or above.

If you had gestational diabetes in a previous pregnancy, your midwife may offer early testing at booking and then repeat at 24 to 28 weeks if the initial result is normal.

Second Trimester: Repeat Red Cell Antibodies (28 Weeks)

If you are Rh-negative, a repeat antibody screen is performed at 28 weeks, just before you receive your routine anti-D injection. This checks that you have not already developed antibodies against Rh-positive red blood cells.

Third Trimester: Repeat FBC (28 Weeks and Beyond)

A repeat full blood count is usually taken at around 28 weeks to check for anaemia. During pregnancy, your blood volume increases by up to 50%, which naturally dilutes haemoglobin. NICE recommends a haemoglobin level of at least 105 g/L at 28 weeks. If your haemoglobin is low, iron supplements will be prescribed and a follow-up test arranged.

Some maternity units also perform a further FBC at 34 to 36 weeks, particularly if previous results were borderline or if you are at increased risk of blood loss during delivery.

Anti-D Injections for Rh-Negative Women

If your blood group is Rh-negative (which applies to approximately 15% of the UK population), you will be offered anti-D immunoglobulin injections. These prevent your immune system from producing antibodies against your baby's red blood cells, which could cause haemolytic disease of the newborn in this or future pregnancies.

Anti-D is usually given as a single dose at 28 weeks or as two doses at 28 and 34 weeks, depending on local protocol. Additional doses may be needed after potentially sensitising events such as bleeding, trauma to the abdomen, or amniocentesis.

Group B Streptococcus (GBS)

Group B streptococcus (GBS) is a bacterium carried by approximately 20 to 30% of women in the UK. While it is usually harmless in adults, it can be passed to the baby during birth and, in rare cases, cause serious infection.

The NHS does not currently offer routine GBS screening, but if GBS is detected incidentally during pregnancy (for example, in a urine culture), you will be offered intravenous antibiotics during labour. Many parents choose to have a private GBS swab test at 35 to 37 weeks for reassurance.

Getting Tested at Home

Pregnancy comes with a lot of appointments. If you prefer the convenience of having some of your blood tests done at home, a mobile phlebotomist from Lola Dispatch can visit you for routine monitoring bloods, NIPT sample collection, or additional private tests.

This can be especially helpful in the later stages of pregnancy when travelling to appointments becomes more tiring. All Lola Dispatch phlebotomists are DBS-checked, qualified, and experienced in antenatal blood collection. Book a home blood test at a time that works for you.