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It can be incredibly scary to find out you have been sensitized, but the most promising information we can share is that with proper monitoring and care the overwhelming majority of alloimmunized pregnancies have a happy ending and all the excitement of getting there becomes a distant memory.   You will find a ton of support by joining the online groups recommended on our Facebook Page .

You may be wondering how you became sensitized or even how a sensitization to red blood cells happens. The simple answer is its similar to how we make antibodies to fight off a virus after exposure.  Everyone has many different antigens on their red blood cells.  You receive one copy from each parent and so we all have a different combination of these antigens as part of our DNA. When you are exposed to an antigen you do not have your body sometimes responds just the same way it would to a virus and creates antibodies to the "invading" antigen. 

There are multiple ways to become sensitized. One way is during a previous pregnancy. Some common ways that can happen during pregnancy are if you had a silent bleed, even so small you don't even realize it happened, trauma like a fall or car accident, or some blood mixing during the birth of your last baby.  Another way is if you have had a blood transfusion. Some people never know when their sensitizing event is and that's okay, try not to let that get to you. 

When you first find out you have antibodies there are some questions you want to ask your provider.  You will want to know which antibody or antibodies you have and also what their titers are.  There are many types of antibodies, but you may have heard of anti D because of the prophylaxis injection that is given to rh negative (D negative) mothers, most commonly rhogam.  If you are sensitized to the D antigen you will no longer require rhogam as you now have the antibodies that rhogam is meant to prevent.  However if you are an rh negative blood type and the antibody you have is NOT D then you should continue to receive rhogam after any potentially sensitizing events and also prophylactically at 26-28 weeks gestation to prevent the formation of anti D.  You also will want to find out if your baby has the antigen you have an antibody to.  If your baby doesn't have the corresponding antigen then there is no danger to your baby from the antibodies in your blood!  

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Antigen negative mother pregnant with antigen positive fetus. Positive red blood cells MAY cross the placenta.

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Antigen positive blood has crossed the placenta and mother has begun to create antibodies.  The fetus is now at risk if developing HDFN.

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Antibodies have crossed the placenta and attached to the fetus's red blood cells.  The fetus is now effected by antibodies, but to what extent is unknown.   

Will my baby have positive blood? Below is a visual to help explain how antigens are inherited and expressed

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There are a few ways to go about finding out whether your baby has the antigen.  The first step is if you are positive on paternity then you will have your baby's father have a simple blood test.  They will do an antigen typing with zygosity testing (also called a phenotype for the specific antigen).  This blood test will tell you if your baby's father has one or two copies of the antigen.  Two copies means he is homozygous and your baby is 100% going to have the antigen and you will move on to the next steps which is monitoring and treatment.  If your baby's father has one copy this is called being heterozygous and there is a 50/50 chance that baby will inherit the antigen.  The noninvasive way to determine if baby has the antigen if paternity is unknown or if the father is heterozygous is through cell free fetal DNA testing.  This can be done for many antigens now, but not all and often insurance in the US does not cover the costs up front.  Many patients do have success having the cost reimbursed though and in other countries it is often included in their universal healthcare!  The other options to determine antigen status are CVS (chorionic villus sampling) which is not recommended for sensitized women due to the risk of additional blood mixing or an amniocentesis.  If you do not wish to have invasive testing and noninvasive testing is not an option for you then you would treat your pregnancy as if the baby was positive and test baby after birth.  

Once you know your antibody and its titer you can more easily determine your next steps.  A titer is how many times your blood can be diluted and the antibodies still detected.  These used to be reported in ratios (like 1:8, but most centers are moving to a whole number reporting for simplicity). For all antibodies except for Kell a critical titer is 16.  For Kell the newest recommendation is not use titers and go straight to weekly screening for anemia starting at 16-18 weeks gestation.  Titers are also not used if this is not your first pregnancy affected by antibodies as they are no longer considered an accurate screening tool once you have had a baby that requires treatment.  If your titers are below critical then you will continue with  blood draws every four weeks to monitor their levels through 26 weeks gestation.  At 26 weeks gestation you will begin having your titers drawn every two weeks until delivery.  

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This is an example of an MCA doppler ultrasound.  We often call these MCA scans for short 

If your MoM is between 1.4 and 1.5 we recommend requesting to come back sooner than one week, but this is up to the discretion of your doctor.  If at any time your MoM reaches 1.5 or above your doctor will need to make a plan of treatment for the suspected anemia your baby has.  Some doctors will schedule you for an intrauterine transfusion immediately and some will request that you come back the next day to confirm your high scan.  We always suggest if this is the case you have a plan for what will happen the next day if scans are the same or greater.   Since steroids have been shown to falsely lower the PSV we recommend that you ask your provider to make a treatment plan before giving steroids.  We really like this calculator to find your own MoM.

maternal alloimmunization HDFN

You will need simple blood draws to determine

what antibody you have as well as its titer

If at any time your titer is 4 or above we recommend a consultation with Maternal Fetal Medicine (perinatologist) to become an established patient and also to be sure you have found one who is knowledgable in this condition and who you trust.

Things we recommend asking at your MFM consultation:

1. Do you have experience with alloimmunized pregnancies?

2. What titer and how often do you recommend MCA scans?  What about antenatal testing?

3. Do you perform intrauterine transfusions? How many a year and are you willing to share your success rate? Which method of IUT is your preferred method?

4. What if I have a concern over the weekend? 

5. Will you deliver my baby or do you co-manage my care with my obstetrician or midwife?

This is important because if your titers become critical (16) at any time you will need to start weekly MCA doppler ultrasounds and there is the potential for fetal anemia requiring treatment. These are specialized ultrasounds that measure the PSV (peak systolic velocity) of your baby's mid cerebral artery (MCA) in their brain. Your doctor will take the best measurement from these scans and using your baby's exact gestational age they will be able to determine their MoM score. The MoM is the multiples of the median and if it is above 1.5 your baby is likely anemic enough to require an intrauterine transfusion (IUT). You can also calculate your MoM by asking your doctor which PSV from the many scans they are using and entering the information in the anemia calculator on this website.

My baby is Anemic.   Now What?

It can be very scary to hear that your baby is anemic and will need an intrauterine transfusion.  In the hands of a skilled provider though the risk is very low (1-3% depending upon the method used) and once your baby has the donor blood they will feel so much better!  

IUTs (intrauterine transfusions) vary a little depending upon the institution where it is performed.  Before your procedure you will have to give a blood sample so that your provider can have blood cross matched and selected appropriately for your baby.  If your baby is of a viable age (22-24 weeks depending upon institution) you may have a round of steroids in the days preceding the IUT.  Some things that will vary from practice to practice are where an IUT is performed.  It may be in an ultrasound room or a surgical suite. Whether a mother has any form of anesthesia is also often provider dependent and sometimes left up to the moms opinion as well.  The differences can be anywhere from no anesthesia at all, a local anesthetic, an epidural or spinal anethesthetic, or in extremely rare circumstances general anesthesia where you will be asleep during the procedure.  Some moms are fully prepped for a potential cesarean section, while others may just have their abdomen sterilized.  You may also be given an antibiotic during your procedure or IV fluids.  Its okay to ask for something to relax you if you are feeling anxious as being able to be still and calm is important during the procedure.  At some hospitals you will be allowed a support person, similar to how you are often allowed one during a cesarean section. Some providers will automatically use a paralyzing agent and or anesthesia on baby and other providers will only do this if baby's movements are hindering the procedure.  None of these variations are the right or wrong way to do an IUT, what is most important is that your care team is confident and experienced in their method.  

This is an incredible ultrasound video of blood going into a baby's umbilical cord during an IUT courtesy of Dr. Gihad Chalouhi  you can find his instagram by clicking his name or his website at


There are two methods for transfusing the blood to baby.  The first and most common method is called an intravascular transfusion or IVT, statistically this is also the method that has the least risk (1-3% depending on which study you refer to). During an IVT the blood is delivered directly into the baby's blood vessel, most commonly at the placental root end of the umbilical cord.  If this is not possible then blood is sometimes transfused into the hepatic vein, or less commonly into a free loop of cord.  Using the IVT method corrects anemia immediately and is performed successfully from about 17 weeks gestation on.  The second method for transfusing blood to the baby is called an intraperitoneal transfusion and statistically the risk associated with this method is a bit higher (3-5% depending upon which study you refer to). The way this type of transfusion is performed is by inserting the needle through the chest and placing blood into the peritoneal cavity.  With this method the blood is not immediately correcting the anemia, but it happens slowly as the body absorbs the new blood.  This is a useful method for babies too small for an IVT if they are in the rare instance of needing to be transfused before they are large enough and also if there is not a safe way for your provider to access a vein.  Additionally some providers will do a combination of both methods during your IUT.  It is important to discuss the risks and benefits of all techniques and to go into your IUT informed of the methods your provider may use.

The Last Weeks of Pregnancy

Beginning at 32 weeks you will start something called antenatal testing.  This is when you will have twice weekly non stress tests and or ultrasounds called biophysical profiles.  A non stress test is when two monitors are placed on your belly and one monitors contractions and the other monitors fetal heart rate and movement.  Your provider will be watching for both movement and the way the baby's heart responds to movement and any contractions.  During a biophysical profile you will go for an ultrasound where they will measure four things: amniotic fluid, whether baby is practice breathing, baby's movements, and baby's muscle tone and then each of these is given a score to give your provider an idea of your baby's wellbeing.


Above is what the monitors look like during a non stress test.  It is non invasive and painless for mom and baby.  They are the same monitors used during labor. 


There are some ways you can help your baby through this pregnancy.  It is so important to keep all appointments and be very honest with your provider.  If you have any concerns or you feel like you may not be getting good care it is okay to seek a second opinion! Do kick counts! There is a great resource that can help you with this.  You will want to chose a time where you can track your baby's movements every day.  Report any changes in your baby's normal to your provider right away!  A saying that we are very passionate about it "When in doubt, get checked out!" Please never be afraid to go in to be monitored if you are concerned or just have a feeling that something is "off".  

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