Management of an Rh-Isoimmunized Pregnancy
We are caring for a young woman who is Rh-negative and became ‘sensitized’ during her first pregnancy. This probably occurred at the time of an emergency cesarean section for ‘fetal distress’ associated with ‘placental abruption’ (premature separation of the placenta) at which time she was probably exposed to a lot more of her baby’s blood than her provider’s realized. She had no Rh-antibodies found in her blood at the time of that admission and the baby had no complications related to anemia and ‘Rh-disease’ after the delivery. The woman was given Rh-immunoglobulin prior to discharge to help prevent sensitization, but only received the standard postpartum dose (sufficient to eliminate no more than about 15 cc of fetal blood from the maternal circulation) which was, as we shall see, probably, not enough in her case. When a larger than usual fetal-maternal hemorrhage is suspected, maternal blood can actually be screened for FETAL blood cells by a test (Kleihauer-Betke test), the amount of fetal blood in the maternal circulation can then be estimated, and sufficient Rh-immunoglobulin given to help prevent isoimmunization under these circumstances.
When she came in for her initial OB visit with her second pregnancy, she was found to have developed antibodies to TWO antigens of the Rh-system (I am not going into that discussion here, but things are never as straightforward as I might have led you to believe in an earlier post!). She had antibody to both the D and C antigens with ‘titers’ (see previous post for an explanation) of anti-D at 16 and anti-C at 32. The father of the baby (who was also the alleged father of her previous baby) was screened and his RBCs were found to have both the D and C antigens. If he did NOT have these, and he was indeed the father of this baby, then the baby could not have them either and our story could probably end here (even though Jerry Springer or Montel would then have an unhappy couple for their shows!). Because the father had the antigens, we now knew that the baby was at risk for having either or both as well, and was therefore at increased risk for ‘Rh-disease.’
The patient was given the option of having an amniocentesis done to find out for sure if the baby was D and/or C antigen positive, but she declined this test. She was followed with monthly antibody titers and at 22 weeks, the titers were found to have increased to 64 for anti-D and to 132 for anti-C. This rise in antibody titers increased the likelihood that the baby is D and C positive. Either of these antibody titers would be in a range that would increase our concern that the baby could develop complications related to Rh-disease, and the combination of two antibodies raised that concern even further.
In circumstances like this in the past, when we wanted to assess the degree of anemia in the baby of an Rh-sensitized woman, the standard approach was to perform an amniocentesis to determine levels of ‘bilirubin’ (see last post), a breakdown product of hemoglobin that is excreted in the fetal urine (amniotic fluid). We have standard curves for amniotic fluid bilirubin, based on years of experience when Rh-isoimmunization was a common problem, that tell us, fairly reliably, when the baby is at ‘low risk,’ intermediate risk,’ and very ‘high risk’ for being anemic. Based on the level of fetal risk, we could then plan when to check the bilirubin again (another amniocentesis), when to consider a ‘cordocentesis’ (a procedure in which fetal blood is sampled from the umbilical cord), when to consider a fetal transfusion (which can be coupled with a cordocentesis so the blood can be delivered directly into the fetal circulation through the umbilical vein if significant anemia is confirmed) or, even, when to just deliver the baby.
Nowadays, we have another alternative to these invasive diagnostic procedures for screening about which our needle-shy patient was delighted to hear. We have learned that by measuring the peak (systolic) blood flow velocity (PSV) in an artery in the baby’s brain (the middle cerebral artery, or MCA) using Doppler flow techniques (ultrasound), we can also identify, very reliably, the babies at increased risk for severe anemia before they have gone into heart failure. High PSV values correlate very well with fetal anemia, especially before 34 weeks, and this noninvasive technique has a very powerful ‘negative predictive value,’ so that when the baby’s PSV in the MCA is within the ‘normal’ range, it is extremely unlikely that the baby has life-threatening anemia.
We followed her weekly and the PSV steadily rose with advancing gestational age (in excess of the normal increase with advancing gestational age), suggesting some degree of fetal anemia, and then suddenly increased dramatically at 36 weeks. It should be noted that by 36 weeks the ‘positive predictive value’ of the PSV decreases significantly (i.e., higher values may NOT necessarily indicate severe fetal anemia), but the degree of the increase was still quite startling. There was no ultrasound evidence of fetal heart failure at this time. An amniocentesis was performed and the amniotic fluid was checked for bilirubin and also assessed for ‘fetal lung maturity’ (another post, another day). The bilirubin was only in the low-intermediate risk zone and the maturity studies were very “immature” suggesting that the baby could be at risk for respiratory problems if delivered at that time. Due to the immaturity of the lung studies, and not feeling pressed to deliver based on the amniotic fluid bilirubin, we gave the patient corticosteroids to help accelerate lung maturation (still controversial at this late gestational age, but proven effective below 34 weeks), and have continued to follow her with fetal heart rate testing and ultrasound. The patient’s due date was established by a first trimester ultrasound, and at this point we will probably simply deliver her by repeat cesarean section at 38-39 weeks.
I will admit up front that, even with four posts, I have left out a fair amount of information regarding Rh-isoimmunization and management during pregnancy. But, I also think I have given you a basic understanding of a very complicated condition and an appreciation for the steps we go through to help minimize risk and ensure a good outcome for both mother and baby. After our patient is delivered, I promise to give you some follow-up of the baby’s course. Thanks for reading....