Routine Antenatal Blood Tests


A haemoglobin test can tell if a person has anaemia. If the haemoglobin level is low, the person needs other tests to identify the cause and the type of anaemia, and to see whether treatment is necessary.


Thalassaemia is a hereditary type of anaemia. Around 8% of the population in Hong Kong are carriers of this disease. The condition is also called thalassaemia minor. A blood test for MCV value can be used as a screening test to check if a person may be a thalassaemia carrier. If the MCV level is low, further tests will be necessary to confirm if the person is indeed a thalassaemia carrier and to differentiate if the person is an alpha(α)-thalassaemia or a beta(β)-thalassaemia carrier. If the couple are both carriers of the same type of thalassaemia gene, their baby will have a one in four chance of suffering from thalassaemia major, a severe form of anaemia. Babies with α-thalassaemia major may die before or soon after birth. Babies with β-thalassaemia major require lifelong regular blood transfusion and medication. Couples carrying the same type of thalassaemia gene can consider prenatal diagnosis to check if the baby is affected by thalassaemia major so that the most appropriate treatment or follow up can be arranged early.

Hepatitis B surface antigen and Hepatitis B surface antibody

Around 10% of the population in Hong Kong are carriers of Hepatitis B virus, but most carriers do not have any symptoms. However, the virus can be transmitted from a carrier mother to her baby during delivery. Hepatitis B surface antigen test (HBsAg) is the test for Hepatitis B carrier status. The newborn of a Hepatitis B carrier mother should receive Hepatitis B immunoglobulin injection to reduce the chance of infection. Mothers who are not Hepatitis B carriers and do not have Hepatitis B antibody (HBsAb) can consider Hepatitis B vaccination after delivery.

Rubella (German measles) antibody

A mother who has received Rubella vaccination or has had the infection before should have protection against the infection, and therefore can avoid passing the virus to her baby during pregnancy. If a mother without immunity contracts the Rubella virus during pregnancy, the infection may cause congenital abnormality in the baby, such as deafness, blindness and heart abnormality. Mothers who do not have Rubella antibody (Rubella IgG) should avoid crowded places or close contact with people who have rashes, and should receive Rubella vaccination after delivery to prevent contracting Rubella in the next pregnancy.

VDRL (Testing for Syphilis infection)

VDRL is a screening test for syphilis infection. If the test shows a positive result, it may indicate syphilis infection but further tests are required to confirm. A mother who is infected with syphilis during pregnancy may miscarry; or the baby may have congenital abnormality such as deafness and blindness. Early treatment can reduce the chance of infection in the baby.

Blood group and Rhesus factor

There are four major blood groups, based on the presence or absence of two antigens – A and B – on the surface of red blood cells. In addition to the A and B antigens, there is a protein called the Rhesus factor, which can be either present (+) or absent (–), creating the 8 commonest blood types: A+, A–, B+, B–, O+, O–, AB+ and AB–. Most Chinese are Rhesus positive (Rh+). If a mother is tested Rhesus negative (Rh–) and her spouse is Rhesus positive (Rh+), there is a possibility of incompatibility between the blood types of the mother and the baby, leading to haemolytic disease of the baby. A Rhesus negative (Rh–) mother needs regular blood checks to see if she needs anti-D immunoglobulin injections during pregnancy and after delivery.

HIV antibody test

This is a test for AIDS (Acquired Immunodeficiency Syndrome) infection, which is caused by infection with the Human Immunodeficiency Virus (HIV). A mother infected with the HIV virus has a 15 – 40% chance of transmitting the virus to her baby during pregnancy, delivery or breast feeding. Treatment during pregnancy and delivery can reduce this chance. It has therefore become a routine to screen all pregnant mothers in Hong Kong. Mothers who have objection against this testing should discuss with the doctor.

Apart from the standard routine antenatal blood tests, there are other optional tests that a mother may wish to consider.


Chickenpox antibody test

Chickenpox is a very contagious disease caused by the Varicella-zoster virus (VZV). It is spread by breathing in droplets from infected people that get into the air when they cough or sneeze, or touching the virus particles from chickenpox blisters. Anyone who has never had the disease or never been vaccinated can be easily infected. If a mother gets infected in early pregnancy, the baby may be affected by various congenital abnormalities including skin lesions, neurological defects, eye diseases, skeletal anomalies or even neonatal death. Mothers with no history of chickenpox infection can have a blood test to check if they have the antibody (VZV IgG). Those who do not have antibodies should avoid contact with people especially children who have skin rashes or blisters; and they can consider the Chickenpox vaccine after delivery to prevent contracting chickenpox infection in the next pregnancy.

Fragile X carrier screening

Fragile X Syndrome (FXS) is the commonest cause of inherited intellectual impairment. It is inherited as an X-linked dominant disorder. All male carriers with full mutation suffer from FXS. They present with learning difficulties (IQ less than 70), behavioral problems and some physical features. About 50% female carriers with full mutation suffer from FXS, with mild to moderate mental disabilities, generally less severe than males.

Fragile X premutation carriers do not have any symptoms when they are young. Female permutation carriers may suffer from early menopause, i.e. no periods before 40 years of age (Fragile X associated Primary Ovarian Insufficiency, FXPOI). Both male and female permutation carriers may develop Fragile X Associated Tremor / Ataxia Syndrome (FXTAS), a late onset neurodegenerative when they reach the age of 50 years. A female premutation carrier can pass the abnormal gene to her offspring, and the premutation gene may become a full mutation in the offspring which leads to FXS in her children. Mothers who are premutation carriers can consider prenatal diagnosis.

If there is a family history of FXS or Fragile X-associated disorders, unexplained mental retardation, developmental delay, autism, menopause before the age of 40; or if the mother has primary ovarian insufficiency, or if she is worried about carrying a baby with FXS, she may consider a blood test to check if she is a Fragile X premutation carrier.

Expanded carrier screening (ECS)

Carriers of recessive genetic diseases are usually healthy and do not have symptoms or have only mild symptoms. They often do not have a family history of the disorder. Therefore, most individuals do not know their carrier status unless they undergo genetic disease screening. Carrier screening is a type of genetic test that analyzes a couple’s DNA to identify whether they are both carriers of a genetic disorder, thus providing specific information about their child’s risk for certain genetic disorders.

The American College of Obstetricians and Gynecologists (ACOG) recommends all women who are already pregnant should be offered carrier screening for spinal muscular atrophy (SMA), cystic fibrosis (CF), thalassemia and haemoglobinopathies. Fragile X premutation carrier screening is recommended for women with a family history of Fragile X-related disorders or intellectual disability suggestive of Fragile X syndrome, or women with a personal history of ovarian insufficiency. Additional screening may also be indicated based on family history or specific ethnicity. The American College of Medical Genetics and Genomics (ACMG) concurs with a similar advice.

Expanded carrier screening (ECS) means that a panel of inherited genetic disorders is screened in a single blood sample. The ECS test offered in our clinic covers 302 genes, and includes all those genetic diseases recommended to be screened by both the ACOG and the ACMG, as well as Fragile X carrier screening.

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