Why it's important to know your blood type before pregnancy

Reviewed by Health Guide Team, 

Written by Kara Earthman 

Reviewed by Health Guide Team, 

Written by Kara Earthman 

last updated: Dec 11, 2018

4 min read

If you’re like most people, you probably saw this article title and thought, “I don’t even know my blood type!” Unless you’re a regular at the blood donation center, have been pregnant in the recent past, or have needed a blood transfusion, chances are you don’t know your blood type. You're definitely not alone, but knowing your blood type is an important part of being your own best health advocate––let's break down why.

The most common blood types are A, B, AB, and O. Your particular blood type is determined by genes (from your parents) and is classified as either A, B, AB, or O based on an antigen, or a type of protein marker, that is present on the cell. Behind the letter, there is also a + or a - sign. This is determined by blood cell antigens, as well. If you are +, this means that your blood cells carry the Rh (rhesus) antigen. If negative, they do not. Blood type does determine what type of blood you can both give and receive (see chart for info).

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Why should you know your blood type?

The most obvious reason why you should know your blood type is in the case of an emergency situation. In any instance where a blood transfusion would need to be performed (if you've lost too much blood, either in a surgery, delivery, or accident), it is vital that the correct blood type be given. Blood type is especially important for women who are pregnant or are considering pregnancy (read on to learn why). Additionally, ongoing research has shown that certain blood types may be at higher risk for developing certain health conditions, and could even possibly impact fertility.

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How does blood type affect pregnancy?

Blood typing will always be done during pregnancy by your provider. To fill us in on exactly how blood type matters in pregnancy, I turned to long-time OB/GYN nurse Ginny Harrington.

First, I wanted to know what exactly Rh is and how the Rh factor (the + or - sign listed behind blood type) impacts pregnancy According to Ginny, “Rh Positive is when proteins appear on the surface of the blood cells. Rh Negative patients are those who do not carry the protein. When a Rh Negative woman gives birth to a Rh Positive baby, this is called Rh incompatibility. This is important, because the Rh Negative mom will treat the Rh Positive proteins found in the baby's blood as foreign, and will make antibodies to attack them (if these blood types mix in any way during the pregnancy). This is called Rhesus Disease, which can cause anemia, severe jaundice, and possible fatal conditions in the newborn.”

In addition to Rhesus Disease, there is also a condition called ABO incompatibility. This can happen when mom’s blood type is different than baby’s (if mom is blood type O, and baby is type A, B, or AB; if mom is blood type A and baby is AB or B; if mom is blood type B and baby is A or AB). If the two different blood types mix, mom’s blood can occasionally develop antibodies that fight baby’s. This attack can result in jaundice in the newborn. Enter an injection, Rhogam, that can be given during pregnancy to prevent this from happening. “This injection provides “temporary antibodies" which eliminate the immune response on the mother's part”, says Ginny. Rhogam is typically given to Rh- pregnant women between 26-28 weeks of pregnancy and again after delivery.

Generally speaking, mom’s blood and baby’s blood don’t mix during pregnancy. However, there are a few instances in which this can happen. Examples of when mixing can occur include car accidents that cause placental bleeding, undergoing a Chorionic Villus Sampling (CVS) or amniocentesis (both uncommonly used procedures that remove a small amount of amniotic fluid or placental tissue from the uterus using a thin needle), falls, and an ectopic pregnancy (a rare but dangerous type of pregnancy that can occur in the fallopian tube).

How does blood type affect overall health, including fertility?

Research is ongoing in regards to how our blood type can impact our overall health. Recent studies have found possible links between blood type and risks for cardiovascular disease, certain types of cancer, diabetes, stroke, and even decreased fertility.

If you are more at risk for any of the conditions below, don’t panic! An increased risk does not mean you will develop the disease. It’s important to be aware, however, if you do have an increased risk, to manage it. While blood type is a risk factor that is not under your control, plenty of risk factors for illnesses are, like diet, exercise, smoking and weight). Here are some examples of risks associated with certain blood types.

  1. Cardiovascular disease: A study performed in 2012 found that those with blood type AB have a 23% higher risk of developing cardiovascular disease compared to people those with blood type O (who have the lowest risk of all the blood types). The link between blood type and cardiovascular risk is not clear; more studies are needed in order to better understand the connection. But it’s important to focus on risk factors that are in your control, such as maintaining healthy cholesterol levels and blood pressure.

  2. Cancer: The American Cancer Society lists type A blood as being a risk factor for stomach cancer. While the link remains poorly understood, there are steps you can take to help prevent the disease, including a healthy diet, adequate exercise, maintaining a healthy weight, and avoiding tobacco.

  3. Diabetes: A 2015 study found that those with type O blood were at a decreased risk of developing type 2 diabetes. As with many of these health risks, the why is not so clear (yet). And if you do have type O blood, it doesn’t mean you won’t develop type 2 diabetes. Everyone, no matter the blood type, should maintain a healthy lifestyle to help prevent type 2 diabetes from developing.

  4. Stroke: Those with type AB blood may be at a higher risk for stroke than those with other blood types, according to a 2014 study. This is believed to be due to higher levels of a clotting protein (why that clotting protein is linked to type AB blood, we’re not sure). Worried? Read up on ways to decrease your risk of stroke here.

  5. Fertility: a small study (544 women) performed did suggest that women with blood type O may be at a higher risk for what’s called “diminished ovarian reserve.” More studies are needed in order to better understand this, so if you have blood type O, don’t panic just yet.

Knowing your own blood type is important for a myriad of reasons. Even if you’re not expecting, learning what you can about your body gives you power over your own health, allowing you to be your best advocate. While blood type is not a health factor you can control, knowing your risks better arms you to focus on those risk factors you can. From emergencies to fertility, pregnancy, and beyond, blood type plays a role in both our current and future health. Who knew your blood type could wear so many hats? Now you do!


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

December 11, 2018

Written by

Kara Earthman

Fact checked by

Health Guide Team

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