Pregnancy Guidelines

A simple blood test can check:

• Antigen status (does he have it?)
• Zygosity (does he carry one or two copies?)

If your antibody is anti-D (Rh), the baby’s father may need a genotyping test to see what type of Rh genes he carries. For other antibodies, a test called a phenotype (also known as antigen typing) is usually done. This test can show whether the father has one copy or two copies of the antigen, which helps estimate the chance of the baby being affected.

What the results mean:

• Two copies = baby will have the antigen
• One copy = baby has a 50% chance

At or after 10 weeks’ gestation, a non-invasive blood draw from the mother can be used to check if the baby inherited the antigen. This is especially helpful when the baby’s father carries only one copy or when paternity is uncertain.

• Safe for baby
• Can detect many antigens (not all)
• May not be covered by U.S. insurance, but reimbursement is often possible

If testing isn’t available, your provider should monitor your pregnancy as if the baby is antigen-positive to be safe.

In some cases, a needle is carefully inserted into the amniotic sac to withdraw a small amount of fluid for testing. While this is the most invasive option, it’s considered the gold standard for confirming certain diagnoses.

A titer shows how active your immune system is by measuring how diluted your blood can be before antibodies are undetectable. Higher numbers = more antibodies.

Most antibodies: A titer of 16 or higher is considered critical and should prompt a referral to an MFM for weekly MCA Doppler scans.

Anti-Kell: Titers aren’t reliable, so monitoring should begin with MCA scans (ultrasound assessment of the middle cerebral artery peak systolic velocity) around 15 to 18 weeks.

Once your titer reaches the critical level, you won’t need further titer draws.

If you’ve had a previously affected critical pregnancy: Titers are no longer reliable; monitoring should begin with MCA scans.

If below critical:

• Every 4 weeks until 26 weeks
• Every 2 weeks until delivery

If Titers Become Critical

Once your antibody titers reach a critical level, your MFM will begin weekly MCA Doppler ultrasounds to monitor for signs of fetal anemia. These ultrasounds measure the blood flow in your baby’s brain. A faster flow can be an early sign that your baby may be becoming anemic.

Your doctor will calculate a value called the MoM (Multiples of the Median).

If your MoM is greater than 1.5, it usually indicates that your baby is developing moderate to severe anemia. An intrauterine transfusion (IUT) may be needed soon. There can be false positives, so repeating the test the next day or two to confirm the findings is a reasonable option.

Your doctor might schedule the IUT right away or repeat the scan within 24 hours to confirm before proceeding. Every case is different, so having a clear plan with your care team is important.

Ask your doctor for the PSV (Peak Systolic Velocity) reading so you can use one of our recommended MoM calculators to better understand your baby’s results and what they mean.

Starting at 32 weeks, most patients with red cell antibodies begin regular checkups:

1. Non-Stress Test (NST):

• Monitors baby’s heart rate & movement
• Two monitors placed on your belly
• Lasts about 20–40 minutes

2. Biophysical Profile (BPP):

• An ultrasound that checks:
  • Amniotic fluid
  • Baby’s breathing movements
  • Kicks & stretches
  • Muscle tone

These tests help your doctor make sure your baby is doing well in the final weeks of pregnancy.

If anything looks concerning, your team can act quickly. That’s why this close monitoring matters.

In rare but high-risk cases, such as when a mom has had a previously severely affected or hydropic baby, your care team may recommend starting treatment early, even before MCA Doppler scans can be used.

Two options your doctor might consider are:

• IVIG (Intravenous Immune Globulin): A weekly infusion that helps block your immune system from attacking the baby’s red blood cells.
• Plasmapheresis: A procedure that removes harmful antibodies from your blood, like a medical filter.

These treatments are sometimes used as early as 12–14 weeks to delay or reduce the need for transfusions and to give your baby more time to grow before any procedure is needed.

Not all pregnancies require this, but if your antibody levels are very high or you’ve had a severely affected baby in the past, it’s something to ask about early.

When a baby develops severe anemia due to maternal alloimmunization, an intrauterine transfusion (IUT) may be needed to replace red blood cells and protect the baby’s health. There are two main types of IUTs, and understanding them can help you feel more prepared and confident going into the procedure.

1. Intravascular Transfusion (IVT)

• Most commonly used and safest option (with a 1–3% complication rate in experienced hands)
• Donor blood is transfused directly into a blood vessel, usually at the base of the baby’s umbilical cord
• Works quickly, making it the preferred method when possible

2. Intraperitoneal Transfusion (IPT)

• Used when IVT isn’t possible due to baby’s position or other factors
• Blood is transfused into the baby’s abdominal cavity, where it is absorbed more gradually
• Some providers may use both IVT and IPT together during the same procedure, depending on what’s safest and most effective

3. Questions to Ask Your Provider

• What are the specific risks and benefits of each transfusion method for my baby?
• Which method do you recommend and why?

Every baby and pregnancy is different, and your medical team will tailor their approach to your unique situation. Don’t hesitate to ask questions—understanding your options is a powerful part of advocating for your baby’s care.