In contrast, the tooth most mesial in each region seemed to be more genetically stable. A subsequent theory hypothesised the teeth at the end of each region were possibly "vestigial bodies" that became obsolete during the evolutionary process. At present, it has been theorised that evolutionary change is working to decrease the human dentition by the loss of an incisor, premolar and molar in each quadrant. According to Vastardis (2000), the size of jaws and number of teeth seem to decrease along with human evolution.Theories focusing on anatomical principle, hypothesised that specific areas of the dental lamina are especially prone to environmental effects during tooth maturation. Svinhufvud et al. (1988) suggested that teeth that were more prone to absence developed in areas of initial fusion of the jaw. For instance, maxillary lateral incisors originate where the lateral maxillae and medial nasal bone processes fuse. In contrast, Kjaer et al. (1994) suggested regions where innervation developed were more sensitive than areas of fusion. Commonly affected regions were found to undergo innervation last, this might imply the developmental relationship between nerve and hard tissue. It is thought to be local nerve development that affects tooth agenesis rather than global development, as brainstem anomalies have not been seen to affect tooth development.Presently, the role of polygenic and environmental factors on hypodontia is recognised in most theories. Environmental factors
Environmental factors can be classified into two main groups, invasive and non-invasive.
See also
Birth control pill formulations
Ethinylestradiol/drospirenone
Estradiol/drospirenone
Ethinylestradiol/drospirenone/prasterone
List of combined sex-hormonal preparations

References
External links
"Drospirenone mixture with ethinylestradiol and levomefolate calcium". Drug Information Portal. U.S. National Library of Medicine.