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What is the RH Factor?

Among the early blood tests a pregnant woman must endure is the Rh, or Rhesus, factor test which is a test to see if the woman is Rh Negative or Rh Positive, which can be an indicator of hemolytic disease.

When the mother has a negative Rhesus factor and the father has a positive Rhesus factor, it can result in a fetus that is positive Rh factor. Even though the blood systems of the mother and baby are separate while the baby is in the womb, there are times when the baby’s blood can enter the mother’s body. When this happens it can cause the mother’s negative Rh factor blood to naturally create antibodies against the baby’s positive Rh factor blood. This results in the body to treating the baby as it would an infection in the mother’s body, instead of the body recognizing that the baby is supposed to be there.

When the mother and baby have opposite RH factor blood, the mother will become sensitized and her body will begin making antibodies which will attack the baby’s positive Rh positive factor blood, causing the breakdown of red blood cells in the baby which can lead to anemia. In severe cases where this happens while the baby is growing in the womb, it can lead to illness, brain damage and even death of the fetus.

The good news is that, through early blood tests, the dangers of the Rh negative factor in women can be prevented. If a woman is found to have RH negative and the father to be RH positive, the woman can be given a shot of Rh immunoglobulin which is a blood product given through an injection, normally in the buttocks of a woman. The injection suppresses the ability of the woman’s blood to react to the Rh positive red blood cells coming from the baby.

Each shot of immunoglobulin, also known as Rhogam, lasts about 12 weeks. Some doctors will choose to give two shots, one at the beginning of the second trimester and one at the beginning of the third, at about 28 weeks. When the baby is born, if it is indeed has the Rh positive, the mother will be given another shot of Rhogam to help avoid the possibility of her body becoming sensitized to the Rh positive factor in later pregnancies.

Most women will test as Rh Positive, at which point there is no threat to the pregnancy. However, some women will test as Rh negative. If your blood work comes back showing that you are Rh negative, your doctor or midwife will mention it to you. However, if you have questions or are unsure if you fall into this group of women, make sure you ask your provider for the results of your blood work at your next appointment.

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4 Comments on "What is the RH Factor?"

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6 years 10 months ago

I just found out that I am pregnant but during the last pregnancy i was informed that I was RH- after my son was born, I have not been to the doctor to see how far along I am yet, as I just found out last night about the upcoming arrival, but anyway, I have had a hard time worrying about what could happen until i do go to the doctor.. has anyone had a bad experience with this issue??/ Thank You

6 years 11 months ago

[…] type is so we know whether or not I will need the Rhogam shot to avoid the potential danger of the rH factor. Milestones: 4 months along […]

7 years 5 months ago

I had to get the rhogam shot when i was around 6 weeks pregnant. I was spotting about every other day. Most of the websites i have looked at says that you normally get it when you are 28 weeks. This is my first pregnancy, right now i am 16 weeks. In 2 more weeks i have to get another shot. I just want to know why im getting shots now, unlike other people who are Rh-. Please email me at

8 years 3 months ago


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