ad. 4. Why is it so important to undergo an anatomical examination before 20 weeks of gestation?

1. the ratio of the ultrasound beam width to the size of the fetus is better up to 20 weeks, this allows to apply a wider range of the transducer clearance angles and thereby to reduce the time of an examination.
2. up to 20 weeks the assessment of the markers for genetic disorders is most sensitive in the second trimester; in that case, beside the assessment of the anatomy of organs it is possible to perform their assessment reliably;
3. in questionable cases this gives time to carry out amniocentesis to identify full karyotype before 22 weeks. It allows to plan the further course of the pregnancy.