A prenatal twin ultrasound is also called a sonogram and is a noninvasive diagnostic test. Sound waves are used to create a visual image of your babies, placenta, and uterus. You’ll be able to see for yourself on a computer, together with the sonographer. Ultrasound scans are usually done through the abdomen, but early in pregnancy they’ll be done through the vagina. Read more about detecting twins early.
A basic ultrasound takes about 15 to 20 minutes. A more detailed twin ultrasound scan can take anywhere from 30 to 90 minutes or more.
When will I have my first scan?
If you are in fertility treatment you’ll probably have your first vaginal ultrasound scan around week 6. If the scan reveals that you’re pregnant with twins you should be able to see two hearts beating at this point. All twins can be detected after week 6+0, regardless of whether they share a placenta (monochorionic) or not (dichorionic). If it’s a spontaneous pregnancy, most doctors recommend that the first scan occur when you’re 11-13 weeks pregnant. They’ll due a scan through your abdomen, check that your babies seem healthy and are growing as they should be. They’ll also confirm (or move) your due date.
– All patients who are suspected to have a twin pregnancy or who are at risk should have first trimester ultrasound performed. Pregnancies resulting from assisted reproductive technologies are in the high risk category.
– Every attempt should be made to determine and report amnionicity and chorionicity when a twin pregnancy is identified. It’s important to know about amnionicity and chorionicity to be on the lookout for Twin Pregnancy Complications such as Twin to Twin Transfusion Syndrome and Twin Anemia Polycythemia Sequence.
– 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. A nuchal translucency scan uses ultrasound to assess your babies risk of having Down syndrome and other chromosomal abnormalities, as well as major congenital heart problems.
– 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. Endovaginal ultrasound is a special form of ultrasound developed to examine the pelvic organs.
– 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. A Doppler scan measures the blood flow through the umbilical cord and around different parts of your babies bodies.
– For defining oligohydramnios and polyhydramnios, the sonographer should use the deepest vertical pocket in either sac: oligohydramnios when < 2 cm and polyhydramnios when >8 cm. Low amniotic fluid is termed oligohydramnios, while a high level of amniotic fluid is called polyhydramnios.
– If you’re pregnant with twins who share a placenta, you should have ultrasound surveillance biweekly. This should occur from 16 weeks’ gestation
*Recommendations are based on a Canadian study published in 2017.