Etiologies of Sex-Discordance in Monochorionic Diamniotic Gestations
Authors: Haven Liana , Tobiasz Ana , Lutz Dennis 
Published: July 2024

Purpose: To investigate the possible etiologies of sex discordancy among monochorionic diamniotic gestations through thirty-five known cases and the addition of a new case presented here. Methods and Case: Analysis of all cases (35) with gender discordance published between 2000 and 2022 were tabulated to include the type of discordance, the testing performed, the details of conception and additional notes on each case. In many cases there was also blood chimerism (designated in our table by the letter b). Four cases were removed from a previous review study because the MC/DC twins only exhibited chimerism or other genetic differences and not discordant sex in either phenotype or chromosomes. A 42-year-old G4P30030 was diagnosed by ultrasound with MC/DA twins having discordant genders. Pregnancy had been achieved using in vitro fertilization (IVF) with donor egg. The donor’s age is unknown but less than 35. Two embryos were transferred. The pregnancy was originally diagnosed as dichorionic/triamniotic but later a vanishing triplet left MC/DA twins. NIPS testing after 16 weeks was within normal limits. The patient and partner initially refused gender reveal until NIPS confirmed the presence of Y chromosome and detailed 20-week scan showed two normal appearing fetuses of different genders. Given the discordance, the patient agreed to fluorescent in situ hybridization (FISH) for chromosomes 21, 18, 13, X and Y along with microarray. FISH for fetus 1 showed XY in 92.5% of the 200 cells evaluated while fetus 2 was XX in 100% of the 100 cells analyzed. Chromosomal microarray was noted to have cells identical to fetus 2 present, consistent with chimerism. Healthy twins were delivered by C-section at 31+3 weeks gestation. Unfortunately, neither preimplantation testing, zygosity testing nor any other postnatal testing was performed on either twin. Results: The frequency of discordant and dizygotic monochorionic gestations has escalated as the rate of ART has risen. To reduce the risk of complications with multiples such as twin-twin transfusion syndrome, congenital anomalies, intrauterine growth restriction and increased fetal morbidity and mortality, there has been a steady push towards single embryo transfer if possible. This has been somewhat offset by the technique of “assisted hatching” where a laser is used to open the zona pellucida to assist in implantation but may also lead to increased monozygotic twinning along with fertilization anomalies. Four etiologies have been hypothesized, three involving splitting and one involving fusion of embryos. In the previous review, 31 of the above cases ART was used 81.2% of the time with 83.9% being discordant. Chimerism was present in 90.33% with only 15.4% exhibiting phenotypic genital anomalies. Of the ow 36 cases, at least 80.5% used some form of ART therapy; the method of conception is unknown, or not documented, in three cases. Among the ART methods, 38.46% (n=10) were ICSI, 38.5% (n=10) IVF, 15.38% (n=4) were intrauterine injection (IUI), and 7.69% (n=2) were with clomid. Among the 26 cases, 4 used assisted hatching (15.4%). Due to the complication of twin-to-twin transfusion, chimerism is defined as genetically different cells from more than one zygote within one individual. With monochronicity it can be inferred that chimerism will be present to some degree due to shared placental blood flow so postnatal testing for chimerism is essential. Although the gravity of chimerism within dizygotic twins is not fully understood, it may confer tolerance to foreign antigens and prove beneficial. Chimerism was noted in 27 cases (75%). Most reported cases of discordant gender in MC/DA twins are thought to begin as 46,XY before splitting to fetuses that are 45,X and 46,XY. The mechanism could be either postzygotic nondisjunction or anaphase lag accompanied by twinning secondary to discordant placental morphology. As this report shows however, most of the present cases do not result in a fetus with Turner Syndrome. A second splitting theory involves a somatic mutation in one of the twins. This could result in either a 46,XY or 46,XX twin although this would be exceedingly rare. A third splitting theory involves a 47,XXY zygote resulting in either a 46,XX and 46,XY pair or a 46,XY and 45,X depending on how the split occurs. The fourth etiology occurs at the blastocyst stage which creates a shared chorion or at the morula stage in the case of ART. In other words, fusion of two separate fertilized embryos occurs before implantation, producing a monochorionic pregnancy that is still diamniotic and dizygotic. ART is thought to alter how embryos interact which may promote fusion or adhesion in close proximity. A corollary theory suggests a chorionic fusion in early pregnancy with subsequent disintegration of the intervening septum layers. Further studies may help unravel this mystery as well as the role played by ART. Conclusion: The complexity of MC/DA gestations ideally requires preimplantation genetic testing, amniocentesis for zygosity testing and postnatal testing on each twin. Reimbursement issues often lead to denial of claims and inability to obtain necessary test results, as occurred with this patient. With increasingly molecular testing sophistication it is often possible to take analysis to the next level. For example: “DNA fingerprinting” may determine if discordant phenotypes reflect discordant genotypes or the fusion of two dizygotic embryos.