Table 1.

Follicle classification and diameter in relation to fetal (or bitch) age during which each class are found within the ovary

Classifications	Age (days)	Follicle diameter (mm)	References
Primordial	17–54	0.04 – 0.05	Andersen and Simpson (1973); Tesoriero (1981); Barber et al. (2001); Blackmore et al. (2004)
Primary or preantral	120	0.050 – 0.08	Durrant et al. (1998); Barber et al. (2001);
Secondary or advanced preantral	NA	0.148 – 0.211	Durrant et al. (1998); Barber et al. (2001)
Tertiary or early antral	120–160	0.350 – 0.360	Durrant et al. (1998); Barber et al. (2001).
Advanced antral	240	2–13	Wildt et al. (1977); England and Allen (1989)