The quality of evidence (I-III) and classification of recommendations (A-L) are defined at the end of the "Major Recommendations" field.
Sonographic Determination of Chorionicity and Amnionicity
- All patients who are suspected to have a twin pregnancy on first trimester physical examination or who are at risk (e.g., pregnancies resulting from assisted reproductive technologies) should have first trimester ultrasound performed. (II-2A)
- Every attempt should be made to determine and report amnionicity and chorionicity when a twin pregnancy is identified. (II-2A)
Determining Gestational Age in Twin Pregnancies
- Although the accuracy in confirmation of gestational age at the first and second trimester is comparable, dating should be done with first trimester ultrasound. (II-2A)
- Beyond the first trimester, it is suggested that a combination of parameters rather than a single parameter should be used to confirm gestational age. (II-2C)
- When twin pregnancy is the result of in vitro fertilization, accurate determination of gestational age should be made from the date of embryo transfer. (II-1A)
- There is insufficient evidence to make recommendation of which fetus (when discordant for size) to use to date a twin pregnancy. However, to avoid missing a situation of early intrauterine growth restriction in one twin, most experts agree that the clinician may consider dating pregnancy using the larger fetus. (III-C)
Screening for Anomalies in Twin Pregnancies
Nuchal Translucency and Maternal Age in Twins
- In twin pregnancies, aneuploidy screening using nuchal translucency measurements should be offered. (II-2B)
There are insufficient data to make recommendations on repeat anatomical assessments in twin pregnancies. Therefore, a complete anatomical survey at each scan may not be needed following a complete and normal assessment. (III)
- Detailed ultrasound examination to screen for fetal anomalies should be offered, preferably between 18 and 22 weeks' gestation, in all twin pregnancies. (II-2B)
Screening for Preterm Birth
There are insufficient data to recommend a routine preterm labour surveillance protocol in terms of frequency, timing, and optimal cervical length thresholds. (II-2)
- When ultrasound is used to screen for preterm birth in a twin gestation, endovaginal ultrasound measurement of the cervical length should be performed. (II-2A)
Assessment of Fetal Growth
- Singleton growth curves currently provide the best predictors of adverse outcome in twins and may be used for evaluating growth abnormalities. (III)
- It is suggested that growth discordance be defined using either a difference (20 mm) in absolute measurement in abdominal circumference or a difference of 20% in ultrasound derived estimated fetal weight. (II-2)
Although there is insufficient evidence to recommend a specific schedule for ultrasound assessment of twin gestation, most experts recommend serial ultrasound assessment every 2 to 3 weeks, starting at 16 weeks of gestation for monochorionic pregnancies and every 3 to 4 weeks, starting from the anatomy scan (18 to 22 weeks) for dichorionic pregnancies. (II-1)
- Increased fetal surveillance should be considered when there is either growth restriction diagnosed in one twin or significant growth discordance. (II-2A)
Use of Umbilical Artery Doppler Velocimetry in Twins
Umbilical artery Doppler may be useful in the surveillance of twin gestations when there are complications involving the placental circulation or fetal hemodynamic physiology. (II-2)
- Umbilical artery Doppler should not be routinely offered in uncomplicated twin pregnancies. (I-E)
Assessment of Amniotic Fluid
Although many methods of evaluating the level of amniotic fluid in twins (deepest vertical pocket, single pocket, amniotic fluid index) have been described, there is not enough evidence to suggest that one method is more predictive than the others of adverse pregnancy outcome. (II-3)
- For defining oligohydramnios and polyhydramnios, the ultrasonographer should use the deepest vertical pocket in either sac: oligohydramnios when <2 cm and polyhydramnios when >8 cm. (II-2B)
Diagnosis of Rare Obstetrical Complications Unique to Twins
Referral to an appropriate high-risk pregnancy centre is indicated when complications unique to twins are suspected on ultrasound. (II-2) These complications include:
- Twin-to-twin transfusion syndrome
- Monoamniotic twins gestations
- Conjoined twins
- Twin reversed arterial perfusion sequence
- Single fetal death in the second or third trimester
- Growth discordance in monochorionic twins
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.