ad. 5. How to best schedule 11-13+6 weeks scan at our centre?
The Fetal Medicine Foundation guidelines clearly state the time when this examination should take place: 11 – 13+6 weeks of gestation when the crown rump length (CRL) is 45 – 84 mm. However, patients who would like to undergo this examination at our centre are strongly encouraged to register after 12 weeks, i.e. 12+, not later than 13+3 weeks (which corresponds to 60-70 mm of fetal length).
Points justifying this approach are listed below:
- the test sensitivity for the Down syndrome, basing on our experience, is the highest when a patient completes 12 weeks of pregnancy and it decreases when a fetus reaches the length of 80 mm.
- an appointment at 12+ guarantees that the majority of patients will fit in time with the examination and will get a result with a calculated risk for the Down syndrome and others.
If we refer a patient, and we give the scope of 11-13+6 weeks, we can encounter a situation, when the real gestational age, calculated on the basis of CRL is different from the one indicated by the last menstruation. The fetus might be younger, which means that there will be a need to postpone the appointment, or older and it will be impossible to perform a precise scan with the calculation of most frequent trisomies risk.
- The length between 60-70 mm optimizes the time of the scan because during this time the position of a fetus toward the long axis of the uterus is the most favorable for the examiner.
- An early anatomy scan, including fetal heart assessment is possible in most cases after 12 weeks of pregnancy.
Therefore, for a couple of years, to the question “When should the first trimester ultrasound examination be conducted?” we have been answering after 12 weeks of pregnancy, i.e. 12+ a few days! Not at 11+, nor 13+.
If a patient underwent an ultrasound examination at ab. 7 weeks, the CRL was measured precisely and the gestational age was calculated reliably, the 12 weeks examination may be scheduled more exactly. For instance, CRL is 6.7 mm (6 weeks+ 4 days), which means that the best time to perform the first trimester screening ultrasound will be in 6 weeks time.