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Hemoglobin Barts
Alpha chains are produced after 5-6 weeks of fetal life. When both of the HBA1 and HBA2 genes which code for alpha globins become dysfunctional, the affected fetuses will have difficulty in synthesizing a functional hemoglobin. As a result, gamma chains will accumulate and form four gamma globins known as Hemoglobin Bart (Hb Bart); an irregular type of Hemoglobin. HB Bart can bind to oxygen, but it cannot release oxygen to the tissue because it has a high level of affinity for oxygen. Therefore, this makes it an inefficient oxygen carrier. This will lead to severe intrauterine hypoxia.

The alpha thalassemia disease has both HBA1 and HBA2 gene deletion and it is the most common cause of fetal hydrops. There is a total of four genes and individuals can inherit two copies of each gene from their father and mother. A silent alpha thalassemia carrier occurs when one of the four alpha genes is defected, and individuals can pass the mutant gene onto their offspring. When there is a mutation that affects two genes, then it results in the individual having an alpha thalassemia trait or alpha thalassemia minor. A mutation that affect three genes results in Hb H disease which is a form of alpha thalassemia. If all four genes are defected, then this results in Hb Bart’s hydrops fetalis. Table 1: α represents the presence of α-globin gene and - represents the deletion of α-globin gene.

Hemoglobin Bart’s usually leads to hydrops fetalis syndrome during the mid-trimester of fetal life as it is unable to synthesize α-globin chain. A prenatal diagnosis can be done to help prevent any pregnancy complications that may occur. Parents should be consulted about the risks of having a future child with hydrops fetalis or Hb H disease. Early detection of Hemoglobin (Hb) Bart's disease before the development of hydrops fetalis is crucial because fetuses that develop hydrops fetalis will either be stillborn or may die shortly after birth. There can be early pregnancy termination to prevent serious complications for the baby or mother. Studies shows that in 11 to 14 weeks of gestation, sonographic markers can associate affected from unaffected pregnancies. It was found that the most sensitive marker was CT ratio and MCA‐PSV. Furthermore, there are several tests that can be done to detect Hb Bart's disease. Ultrasound screening is done as a part of early prenatal diagnosis for Hb Bart's disease before the growth of hydropic changes. It is shown that out of the ultrasound markers, cardiac diameter to thoracic diameter (C/T) ratio is the most accurate in predicting HB Bart's disease.

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