Differentiating Types of Twins
In the 1st-trimester twin pregnancies are categorized into different types depending on their presentation in the gestational sac and most importantly to date the twins using crown-rump length (CRL) but also to take a measurement of the nuchal translucency (NT) for the combined screening test for chromosomal abnormalities. The main types of twin pregnancies are DCDA (dichorionic diamniotic), MCDA (monochorionic diamniotic), and MCMA (monochorionic monoamniotic). 2/3 of twins are DCDA with two placentas and two separate amniotic sacs. 1/3 will be MCDA, in which they share a placenta but have two separate amnions. MCMA is presented in a small proportion of twin pregnancies with a single placenta shared by the two fetuses in the same amniotic sac and in the absence of a dividing membrane. In DCDA there is a likelihood of two eggs being fertilized by two sperms or one egg fertilized by one sperm and separated into two embryos, the latter is true for MCDA and MCMA twins. The differentiation of the two subtypes is important in deciding further management and surveillance in later gestation as monochorionic twins are more likely to suffer from complications, such as twin to twin transfusion syndrome (TTTS) and twin reversed arterial perfusion (TRAP), or in extreme cases fetal demise due to highly discordant growth velocities.
The use of U/S as an imaging modality in the first trimester is due to its various advantages. It is an inexpensive, quick examination that is likely to help confirm the viability and intrauterine position of the gestational sac. There is no bioeffect associated with u/s in particular when safety guidelines are adhered to avoid thermal and mechanical effects. It is a non-invasive test that is comfortable for most patients in the first trimester of their pregnancy. Disadvantages are that the quality of the images is most likely to be affected by the mother’s large body habitus (high BMI) and some patients may not be able to follow full preparation of a full bladder to attain the uterus in the best position. In some cases where uterine anomalies such as didelphys, unicornuate or bicornuate uterus are presented it can be difficult to assess even a singleton pregnancy accurately and can further pose problems in twins. The fetal lie or presentation can also affect the adequacy of the image quality in getting a CRL, in which if measured wrongly the subsequent growth velocity curve may be hindered and cause errors in estimating the true gestational age to growth velocity.
Main –The first trimester scans are most likely to be performed between 11 to 14+1 weeks of gestation (from the last menstrual day), CRL should be between 45-84mm for the combined screening to take place, and if above head circumference should be considered to accurately date the pregnancy and to carry out the quadruple test for Down’s. This is the optimum time to assess the chronicity and amnionic of a twin pregnancy as the pregnancy progresses the dividing membrane may be distorted by fetal parts. The lambda sign can be identified with DCDA twins, MCDA with a T-shaped sign and MCMA has no dividing membrane as they share the amniotic cavity and the placenta. Uniformly oval hypoechoic center of the gestational sac eccentrically placed in the endometrial cavity should confirm the pregnancy as an intrauterine with two clearly seen gestational sacs with a dividing membrane for the DCDA/MCDA and two embryos within a single gestational sac as MCMA. If there is an empty gestational sac present with no fetal pole this should be considered as a vanishing twin in which the CRL and NT measurement can be taken for the viable twin to assess any chromosomal abnormalities. However, if there is another sac or fetal pole with no cardiac activity (with fetal pole measuring > 7mm) only the CRL of the viable twin is attained to gain an accurate gestational age of the pregnancy with the NT measurement. A second opinion should be obtained to confirm the non-viability. The combined screening cannot be undertaken for the viable twin in this scenario as there is a likelihood of erroneous levels of maternal hormones in the blood test.
In addition to attaining CRL and NT measurements, the first-trimester scan can alert a sonographer if there is a likelihood of early signs of TTTS. CRL measurement should be similar in both twins indicating a similar gestational age, if highly discordant( more than 8mm difference) then it may have a predictive value in identifying major growth delays in later gestation. Often the biggest CRL is used to date the pregnancy. CRL discordance further should be investigated in MC twins as early signs of TTTS as they are more likely to share a placenta hence increasing the likelihood of the disease. High NT differences in monochorionic pregnancies are also associated with a higher chance of TTTS in later pregnancies. In TTTS there is an imbalance of net blood flow across the placental vascular communications from the fetus (donor) to the other fetus (recipient). It can further cause to form of abnormal vascular anastomoses that can distribute the maternal circulation disproportionally. The donor may have a high likelihood of developing intrauterine growth restriction and oligohydramnios (deepest vertical pocket < 2cm and the recipient may be large in size with mega cystitis with polyhydramnios ( > 8cm deepest pool). However, it can be difficult to predict this in the first trimester as the placenta and the vascular channels are not fully developed before the 16th week of gestation.
Twins may also be presented with lethal structural abnormalities, hence as part of FASP first trimester guidelines, it is important to identify the cranium, limbs, abdomen wall, and cord insertion for both sets of fetuses. Common lethal structural abnormalities that are presented in the 1st trimester can be anencephaly, cystic hygroma, hydrocephalus, microcephaly or open spina bifida. Early detection can be beneficial in counseling to early termination, however major abnormalities are optimally detected in the 18-20 weeks scan in many pregnancies. Further to that, other features such as prominent midgut herniation may be still observed in some fetuses at 12 weeks. Midgut herniation is a physiologically normal variant till 12 weeks where the abdominal wall herniates into the umbilical cord. This can be fully assessed by another scan at 14-16 weeks to rule out any chance of gastroschisis or omphalocele. Anencephaly is the absence or incomplete development of a major portion of the brain and skull with a frog eye-like appearance and this should be ruled out by assessing the cranial vault and the two hemispheres in a transverse section in both fetuses. Upper and lower limbs should be seen for both fetuses and the cord insertion into the stomach should be confirmed in line with the midgut herniation appearance. The location of the placenta in terms of the internal os of the cervix should be confirmed. The importance in identifying the correct location will help differentiate the twins in subsequent examinations. Usually, in the first trimester, the twins are identified as twin A or B in terms of the uterine position, and the fetus closest to the cervix is identified as the presenting twins. This is important to document well as discrepancies can further lead to the erroneous assumptions of the individual growth of the fetuses in later gestations. Finally, the scan should confirm any adnexal pathologies (i.e. fibroids or corpus luteal cysts) in the mother’s uterus.
Conclusion
U/S has a significant role in differentiating types of twins presented in a pregnancy where there are multiple embryos. The first-trimester scan can also confirm the location as an intrauterine pregnancy and can identify the correct gestational age for the fetuses that can be used in to assess the growth velocity in later gestations. It can also help to identify any further risks that can impose the fetal well-being and possibly refer pregnancies that are highly suspicious of developing TTTS to fetal medicine specialties. U/S as a modality can be useful in the diagnosis of major structural abnormalities presented in the early stages of pregnancy without any harmful bioeffects to the fetuses which may be used to counsel parents for intrauterine therapy to further karyotyping tests, such as CVS and amniocentesis.
