Organs develop out of primordia (tissue in its earliest recognizable stage of development). The developmental processes of the face and jaw structures originate from different primordia:

Unpaired frontonasal process
Paired nasomedial and nasolateral processes
Paired maxillary processes and mandibular processes

The formation of the frontonasal process is the result of a complex signaling system which begins with the synthesis of retinoic acid (a vitamin A metabolite). This is needed to set up the facial ectodermal zone. This zone makes signaling molecules that stimulate the cell proliferation of the frontonasal process. A midfacial defect will occur if this signaling pathway is disrupted. It is suggested that the absence of this pathway will lead to the formation of a gap, and that when the pathway is working too hard, excessive tissue will be formed. FND consists of various nasal malformations that result from excessive tissue in the frontonasal process, which results in hypertelorism and a broad nasal bridge.Between the 4th and 8th week of pregnancy, the nasomedial and maxillary processes will fuse to form the upper lip and jaw. A failure of the fusion between the maxillary and nasomedial processes results in a cleft lip. A median cleft lip is the result of a failed fusion between the two nasomedial processes. The palate is formed between the 6th and 10th week of pregnancy. The primordia of the palate are the lateral palatine processes and median palatine processes. A failure of the fusion between the median and lateral palatine processes results in a cleft palate.
However, only three cases of FND have been diagnosed based on a prenatal ultrasound.Other conditions may also show symptoms of FND. For example, there are other syndromes  that also represent with hypertelorism. Furthermore, disorders like an intracranial cyst can affect the frontonasal region, which can lead to symptoms similar to FND. Therefore, other options should always be considered in the differential diagnosis. Types
Pai syndrome
The Pai Syndrome is a rare subtype of frontonasal dysplasia. It is a triad of developmental defects of the face, comprising midline cleft of the upper lip, nasal and facial skin polyps and central nervous system lipomas. When all the cases are compared, a difference in severity of the midline cleft of the upper lip can be seen. The mild form presents with just a gap between the upper teeth. The severe group presents with a complete cleft of the upper lip and alveolar ridge.Nervous system lipomas are rare congenital benign tumors of the central nervous system, mostly located in the medial line and especially in the corpus callosum. Generally, patients with these lipomas present with strokes. However, patients with the Pai syndrome don’t. That is why it is suggested that isolated nervous system lipomas have a different embryological origin than the lipomas present in the Pai syndrome. The treatment of CNS lipomas mainly consists of observation and follow up.Skin lipomas occur relatively often in the normal population. However, facial and nasal lipomas are rare, especially in childhood. However, the Pai syndrome often present with facial and nasal polyps.