Introduction:
Prenatal genetic testing and identification impart a variety of unique challenges when twin pregnancies occur, and the process is generally more intricate than when singleton pregnancies (having a single baby when pregnant) occur. Screening for trisomy 21 during pregnancy in twins suggests a greater likelihood of poor newborn outcomes, especially because preterm births are more common. There are still a lot of significant obstacles to overcome, even in light of the rise in twin pregnancies and the rising interest in the prenatal evaluation of multiple gestations.
Which Screening Test Is Suitable for Twins During Pregnancy?
Nuchal Translucency:
Nuchal translucency (NT) is the ultrasound representation of an accumulation of fluid beneath the skin below the fetal neck during the first trimester of pregnancy. The measurement of nuchal translucency (NT) can be utilized for screening in multiple and twin pregnancies since trisomy 21 twin pregnancy fetuses may exhibit thickening of the NT. When compared to dichorionic twins (a kind of twin pregnancy in which the amniotic and chorionic sacs are unique to each twin), monochorionic twins (twins who are identical and share the same placenta) typically exhibit a larger percentage of enhanced translucency and a much higher average nuchal translucency thickness.
However, it is crucial to take this feature into account to prevent needless invasive procedures for confirmation, as there is no proof of an increased prevalence of trisomy 21 in monochorionic twins. Instead, cardiac problems, deformities, and twin-to-twin transfusion may be indicated by an increased NT in monochorionic twins. Since dizygotic twins are thought to be dichorionic, the background risk includes each individual NT. It is more probable that a rise in both NT values in a pair of dizygotic twins is due to a hereditary tendency towards enlarged NT thickness than to the infrequent presence of a genetic defect in both fetuses.
Non-Invasive Prenatal Testing (NIPT):
Non-invasive prenatal testing (NIPT) by examination of cell-free DNA (cfDNA) in the mother's blood has demonstrated a high rate of identification for trisomy 21 among other frequent autosomal trisomies. The non-invasive prenatal testing (NIPT) is used in monozygotic pregnancies (a fertilized egg splits into two), in which the twins have the same genome. It seems to be more effective than in singleton pregnancies, or at least as effective.
If the fetal fraction of the afflicted fetus in a dizygotic pregnancy (when two distinct sperm fertilize two eggs) inconsistent for trisomy is less than four percent—the percentage required in a singleton pregnancy to identify aneuploidy (abnormal number of chromosomes)—the results may falsely suggest a low risk. Consequently, it has been suggested that the lower proportion of the two fetuses should be assessed in the evaluation of the likelihood of aneuploidy rather than the total fetal fraction in order to establish a valid diagnosis in a dizygotic pregnancy.
What Are the Invasive Techniques Involved in Twin Pregnancies?
There are certain unique characteristics specific to twin pregnancies that must be taken into consideration while performing invasive prenatal diagnostic procedures. Chorionic villus sampling is a prenatal test that includes extracting a sample of placental tissue to test for chromosomal abnormalities and certain other genetic disorders. A procedure called amniocentesis is done to remove a little sample of amniotic fluid for analysis.
Prioritizing the differentiation between monochorionic and dichorionic twins is crucial in this discipline as it provides a clear indication of which of the two approaches to pursue. When it comes to chorionic villus sampling (CVS), possible cross-contamination and sample inaccuracy are major concerns. The technique used can be transabdominal (TA), transcervical (TC), or combination. Considerations for amniocentesis include single versus double uterine entry, sample inaccuracy and cross-contamination, and dye usage. Compared to singleton pregnancies, twin pregnancies tend to have a greater percentage of loss of pregnancy due to the intrusive features of the process. It is crucial to remember that twin pregnancies have a higher baseline risk of pregnancy loss, thus it is not possible to precisely compare the outcomes of singleton and twin pregnancies.
Based on a meta-analysis of nonrandomized cohort studies, the overall loss of pregnancy after amniocentesis and CVS in pregnancy with twins may be interpreted similarly. Remarkably, no CVS study has taken chorionicity into account while calculating the rate of miscarriage following an invasive test.
Monochorionic Conception:
Given that monochorionicity and monoamnionicity are markers of monozygosity, it is anticipated that the two fetuses will have the same karyotype (an illustration of a cell's entire chromosomal complement). Under this supposition, it is reasonable to justify a single amniotic fluid sample at a later gestational age or a single chorionic villus sample during the first trimester. This holds particular significance as it is apparent that inserting a needle twice into the mother's belly to collect fetal material for prenatal evaluation carries more hazards than just one insertion.
Monozygotic twin cases of heterokaryotypia, or discordant karyotype, are becoming more common. These occurrences could be the result of mistakes made during mitosis or twinning events, like postzygotic non-disjunction. In these situations, it is crucial to sample both fetuses and to definitively link each sample to a specific fetus. Amniotic fluid should ideally be sampled from each twin sac because chorionic villus sampling cannot provide such assurance. Microsatellite marker genotyping is employed to verify the monozygosity of discordant twins, and molecular karyotyping may be advantageous for cytogenetic investigations when discordant traits exist between monozygotic twins.
When a monochorionic twin pregnancy occurs, extracting amniotic fluid from both sacs is not usually an easy task. In fact, in about 20 % of cases, twin-to-twin transfusion syndrome may cause a significant reduction in the amount of amniotic fluid in one of the two sacs. This could lead to large or moderate differences in the fluid between the sacs, which could be linked with or unrelated to a selective intrauterine growth restriction. This could make it more difficult to sample both amniotic sacs. Sometimes, amnioreduction can be used to acquire amniotic fluid sampling for invasive diagnosis during an invasive surgery for the treatment of twin-to-twin transfusion syndrome prior to treatment with lasers.
Dichorionic Conception:
About 10 percent of dichorionic twin pregnancies are likely to be monozygotic, with the majority being dizygotic. As such, the type of placentation effectively determines the diagnostic approach to be used. Through the use of chorionic villus sampling during the first trimester, each fetus can be accurately sampled. The exclusive chance of cross-contamination with CVS during a twin pregnancy is roughly one percent. An intra-amniotic dye may be utilized during amniocentesis.
Typically, chorionic villus sampling takes place in the 10th and the 12th week of pregnancy. Various means of sampling, such as transabdominal, transcervical, and transvaginal, are available to accomplish the sampling of both placentas; the latter two are mainly employed in conjunction. The transabdominal approach has the benefit of being able to be used later in pregnancy, while the transcervical and transvaginal modes can only be used until the 13th week of pregnancy. The placental material that was sampled can be utilized for molecular karyotyping, genotyping, and cytogenetic tests to rule out monozygosity or accidentally repeating the sampling of the same fetus. In the event that one fetus exhibits indications of a genetic defect and the results are inconsistent, first-trimester selective embryo reduction is a treatment option made possible by the type of placentation.
What Is Selective Termination of Pregnancy?
A pregnant woman may request the selective termination of the affected fetus if she is diagnosed with an inconsistent karyotype and there is a significant abnormality present in one of the twins. An additional factor could be the existence of a baby with an anatomical abnormality, and this could raise the chance of a poor perinatal prognosis for the entire length of the pregnancy. Though most people believe selective termination to be a relatively safe procedure, there are a few important factors to take into account:
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The probability of loss of pregnancy is associated with the procedure itself.
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The potential for maternal coagulopathy as a result of the continued presence of a nonviable fetus.
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The emotional impact on a pregnant woman who continues to bear a nonviable fetus for an extended amount of time during the pregnancy.
The method of selective reduction varies based on the pregnancy's chorionicity. One method needed for the fetal reduction in monochorionic pregnancies is to ablate or cut off the afflicted fetus's umbilical cord's blood supply. It is not viable to utilize KCl injections in monochorionic pregnancies because of the existence of vascular anastomoses, as this would result in the death of the co-twin; alternative methods have been investigated for this purpose. Bipolar cord coagulation (BCC) and radiofrequency ablation (RFA) are the two techniques that are being utilized.
Selective reduction can be carried out transabdominally in the second trimester and transvaginally or transabdominally in the first in dichorionic twins. As part of the procedure, a 21-gauge needle directed by a U.S. probe is used to deliver potassium chloride (KCl) into the region of the fetal heart. Within a minute following the injection, the heart stops beating. Ultrasound monitoring is required for the activity, and in rare instances, repeat injections may be required.
Conclusion:
Although it presents numerous problems, the field of twin prenatal diagnosis and screening is noteworthy. There is an acute demand for better, less invasive procedures in this particular field. Above all, nevertheless, the most precise determination of the zygosity and chorionicity of the gestation should be the top focus.
