Expecting moms with Rh-negative blood are at risk for Rh sensitization if their baby has Rh-positive blood.  When your baby’s Rh-positive blood mixes into your Rh-negative blood, your immune system will create antibodies, or types of immune cells, to attack your baby’s red blood cells. This reaction by your immune system is called Rh sensitization. If left untreated, Rh sensitization can cause serious problems in your baby, including brain damage, heart failure, anemia (deficiency of red blood cells), jaundice (a liver condition causing yellowing of the baby’s skin and eyes), stillbirth, or death after birth.  Luckily, Rh sensitization is both treatable and preventable. Your first baby is typically not at risk from this reaction because the mixing of your baby’s blood and your blood does not usually occur until delivery. It also takes time for the immune system to create antibodies. If you have Rh-negative blood, it is usually your second pregnancy and any later pregnancies that are most at risk. 
What is Rh factor?
Your blood type consists of an ABO type and an Rh type. Both the ABO type and the Rh type are determined by the presence or absence of certain antigens, which are sugars or proteins, on your red blood cells. [3,4] The ABO type involves the presence of either A antigens, B antigens, or no antigens in the case of the O blood type. This determines whether you have type A, type B, type AB, or type O blood. The Rh type is either Rh-positive or Rh-negative and determines whether you have a positive (+) or negative (-) symbol after your ABO blood type. You are Rh-positive if you have Rh factor (a “D antigen” on the red blood cell’s surface), and your type is Rh-negative if you do not have Rh factor. You can read more about Rh factor here.
How do I know if I am at risk for Rh sensitization?
You will have a blood test at your first prenatal visit to determine your blood type.  Expecting moms who have Rh-negative blood are at risk for Rh sensitization, but only if their baby has Rh-positive blood. The blood type of your baby can be guessed if you know the Rh type of your baby’s father. Your baby will have Rh-negative blood if both you and your baby’s father have Rh-negative blood. If your baby’s father has Rh-positive blood and you have Rh-negative blood, your baby can have either Rh-positive or negative blood, and Rh sensitization may occur. Your doctor may also recommend that you have an amniocentesis (a test where some of your amniotic fluid is sampled) to determine the blood type of your baby. If you have Rh-negative blood, your doctor may decide to treat you as if your baby has Rh-positive blood to be safe.
How do I know if I have been Rh sensitized already?
Expecting moms with Rh-negative blood have been Rh sensitized if they have antibodies to Rh factor (Rh(D) antibodies). If you have Rh-negative blood, your doctor will continually test your blood for the presence of Rh(D) antibodies.  Once you have these antibodies, treatment may be required to ensure the safety of your baby because your Rh(D) antibodies will cross the placenta and attack your baby’s red blood cells. The disease that this causes in your baby is called Rh disease or hemolytic disease of the newborn.  If Rh disease is severe and causes anemia or jaundice in your baby, he or she will need to be treated immediately, and you may need to deliver your baby prior to your due date.  Your baby may also need a blood transfusion to replace red blood cells that have been destroyed by your body’s Rh(D) antibodies. Blood transfusions may be performed either when your baby is still in the womb after at least the 18th month of pregnancy or after your baby is born. In cases where Rh disease is mild, some babies reach full-term, do not require treatment, and are able to fully recover after birth. Other babies with mild Rh disease may require treatment after birth, but most still recover fully.
How can I prevent Rh sensitization?
Unless the baby’s father is known to be Rh-negative, it is recommended that unsensitized expecting moms have repeated Rh(D) antibody tests at 24 to 28 weeks of pregnancy and receive an injection of Rh immunoglobulin, called Rh immune globulin. [2,5] Rh immune globulin will help prevent Rh sensitization by stopping your immune system from producing Rh(D) antibodies. If you give birth to a baby that is Rh-positive, your doctor will give you an additional dose of Rh immune globulin. Your doctor may also give you Rh immune globulin if other situations occur that could result in the mixing of your blood with your baby’s blood, such as amniocentesis, ectopic pregnancy, or a miscarriage. Rh immune globulin is usually successful for the prevention of Rh sensitization and most babies will not require further treatment.  You can read more about the Rh immune globulin shot here.
Your doctor will screen you during your first prenatal visit to determine your Rh blood type. If you are Rh-negative, the father of your baby should also be tested for his Rh type, or your doctor may recommend an amniocentesis to determine your baby’s blood type. Expecting moms who have been Rh sensitized in the past must take special precautions during their second pregnancy. Rh disease can cause serious health problems in babies, but with proper screening and treatment, this disease is largely preventable.
- WebMD. Rh Sensitization During Pregnancy – Topic Overview.
- March of Dimes (2017). Rh disease.
- Dean L. Blood group antigens are surface markers on the red blood cell membrane. In: Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005.
- U.S. National Library of Medicine. Blood typing. MedlinePlus.
- U.S. Preventive Services Task Force. Screening for Rh(D) Incompatibility: Recommendation Statement. Am Fam Physician. 2005;72(6):1087-1088.
- WebMD. Rh Sensitization During Pregnancy – What Happens.
- Slightham C (2015). Rh Incompatibility. Healthline.