The Society of Obstetricians and Gynaecologists of Canada (SOGC) has released new recommendations for the use of ultrasound in twin pregnancies.
The Clinical Practice Guideline, published in the June issue of the Journal of Obstetrics and Gynaecology Canada, was prepared by the SOGC's Diagnostic Imaging Committee and approved by the society's Genetics Committee and Maternal Fetal Medicine Committee. The authors produced their recommendations following a review of PubMed and the Cochrane Library in 2008 and 2009 for studies involving the use of diagnostic ultrasound in twin pregnancies.
In specific recommendations, the authors advised the following:
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.
Every attempt should be made to determine and report amnionicity and chorionicity when a twin pregnancy is identified.
Although the accuracy in confirmation of gestational age at the first and second trimester is comparable, dating should be done with first-trimester ultrasound.
Beyond the first trimester, it is suggested that a combination of parameters rather than a single parameter should be used to confirm gestational age.
When twin pregnancy is the result of in vitro fertilization, accurate determination of gestational age should be made from the date of embryo transfer.
There is insufficient evidence to make a 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.
In twin pregnancies, aneuploidy screening using nuchal translucency measurements should be offered.
Detailed ultrasound examination to screen for fetal anomalies should be offered, preferably between 18 and 22 weeks' gestation, in all twin pregnancies.
When ultrasound is used to screen for preterm birth in a twin gestation, endovaginal ultrasound measurement of the cervical length should be performed.
Increased fetal surveillance should be considered when there is either growth restriction diagnosed in one twin or significant growth discordance.
Umbilical artery Doppler should not be routinely offered in uncomplicated twin pregnancies.
For defining oligohydramnios and polyhydramnios, the ultrasonographer should use the deepest vertical pocket in either sac: oligohydramnios when less than 2 cm and polyhydramnios when greater than 8 cm.
