Throughout normal pregnancy the fetus swims freely in amniotic fluid, being able to assume any position, presentation, or posture necessary to adapt to intrauterine environment. Fetal activity, an event of little consequence in the absence of placenta prevail, uterine anomalies or fetal malformations, can be easily observed with 2D, 3D and especially by using 4D. The latter allows the visualization of opening the mouth and other subtle movement of special interest in cases of some malformations like arthrogryposis multiple congenital.
Between weeks 28 and 32 pregnancy the fetus adopts a longitudinal position due to the change in uterine shape from spherical to avoid (Reynolds transformation phenomenon). Most fetuses will acquire a chepalic presentation, which is likely to remain up to the time of delivery. A few fetuses will be in breech presentation and this may even during labor.
Most fetuses re in attitude of flexion so that the chin appears to rest in the upper chest, the arms are folded, and the legs somewhat bent at the knees and crossed at the ankle level. Because of this position , it is difficult to observe the face prior to 12 or 14 weeks of gestation. Beyond this gestational age it becomes easier to observe the fetal attitude and features with 2D and even more so with 3D. Beyond the third trimester the fetus has features and attitudes that are identical to those of a child. some fetuses seem to < refuse to be photographed>(figure) allowing to observe their posture details , fingers, hands and articulation. These details are important for the evaluation of malformations that involve the extremities, a subject to which we have dedicated full post.
In our experience, 3D –4D imaged if the fetus are much realistic and of better quality than 2D images and allow clearer visualization of the details that are necessary to arrive at a more accurate prenatal diagnosis of fetal malformations