The quality of evidence (I-III) and classification of recommendations (A-L) are defined at the end of the "Major Recommendations."
Prenatal Screening in Twins
Aneuploidy Risk Estimation in Relation to Maternal Age, Zygosity, and Chorionicity
- All pregnant women in Canada, regardless of age, should be offered, through an informed counselling process, the option of a prenatal screening test for the most common clinically significant fetal aneuploidies. In addition, they should be offered a second trimester ultrasound for dating, assessment of fetal anatomy, and detection of multiples. (I-A)
- Counselling must be non-directive and must respect a woman's right to accept or decline any or all of the testing or options offered at any point in the process. (III-A)
- When non-invasive prenatal screening for aneuploidy is available, maternal age alone should not be an indication for invasive prenatal diagnosis in a twin pregnancy. (II-2A) If non-invasive prenatal screening is not available, invasive prenatal diagnosis in twins should be offered to women aged 35 and over. (II-2B)
Nuchal Translucency in Twins
Fetal nuchal translucency combined with maternal age is an acceptable first trimester screening test for aneuploidies in twin pregnancies. (II-2)
- Chorionicity has a major impact on the prenatal screening process and should be determined by ultrasound in the first trimester of all twin pregnancies. (II-2A)
- When screening is done by nuchal translucency and maternal age, a pregnancy-specific risk should be calculated in monochorionic twins. In dichorionic twins, a fetus-specific risk should be calculated. (II-3C)
Nuchal Translucency Combined With Serum Markers
First trimester serum screening combined with nuchal translucency may be considered in twin pregnancies. It provides some improvement over the performance of screening by nuchal translucency and maternal age by decreasing the false-positive rate. (II-3)
Integrated screening with nuchal translucency plus first and second trimester serum screening is an option in twin pregnancies. Further prospective studies are required in this area, since it has not been validated in prospective studies in twins. (III)
Invasive Prenatal Diagnosis for Twin Pregnancies
Offering Invasive Testing in Multiple Pregnancies
Non-directive counselling is essential when invasive testing is offered. (III)
Single or Double Sampling in Monochorionic Twins
- During amniocentesis, both amniotic sacs should be sampled in monochorionic twin pregnancies, unless monochorionicity is confirmed before 14 weeks and the fetuses appear concordant for growth and anatomy. (II-2B)
Chorionic Villus Sampling
Chorionic Villus Sampling Error
When chorionic villus sampling is performed in nonmonochorionic multiple pregnancies, a combination of transabdominal and transcervical approaches or a transabdominal only approach appears to provide the best results to minimize the likelihood of sampling errors. (II-2)
Management of Twins Discordant For Karyotypical Anomalies
- Prior to invasive testing or in the context of twins discordant for an abnormality, selective reduction should be discussed and made available to those requesting the procedure after appropriate counselling. (III-B)
- Monitoring for disseminated intravascular coagulopathy is not indicated in dichorionic twin pregnancies undergoing selective reduction. (II-2B)
Quality of Evidence Assessment*
I: Evidence obtained from at least one properly randomized controlled trial
II-1: Evidence from well-designed controlled trials without randomization
II-2: Evidence from well–designed cohort (prospective or retrospective) or case–control studies, preferably from more than one centre or research group
II-3: Evidence obtained from comparisons between times or places with or without the intervention. Dramatic results in uncontrolled experiments (such as the results of treatment with penicillin in the 1940s) could also be included in this category.
III: Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees
*Adapted from the Evaluation of Evidence criteria described in the Canadian Task Force on Preventive Health Care.
Classification of Recommendations†
A. There is good evidence to recommend the clinical preventive action.
B. There is fair evidence to recommend the clinical preventive action.
C. The existing evidence is conflicting and does not allow to make a recommendation for or against use of the clinical preventive action; however, other factors may influence decision-making.
D. There is fair evidence to recommend against the clinical preventive action.
E. There is good evidence to recommend against the clinical preventive action.
L. There is insufficient evidence (in quantity or quality) to make a recommendation; however, other factors may influence decision-making.
†Adapted from the Classification of Recommendations criteria described in the Canadian Task Force on Preventive Health Care.