Since the 1960’s it has been believed that increased estrogen levels during the female heat cycle will affect hip joint conformation by making them appear falsely lax (looser).
In 1997, researchers at the University of Pennsylvania decided to test this theory and followed 9 bitches through an entire estrus cycle. During the study vaginal cytology, behavioral signs and physical exam findings were used to identify the different stages of the cycle. Blood samples were obtained during each estrus-phase, from which hormone concentrations were measured, specifically oestradiol-17β and progesterone. Hip x-rays were taken using the OFA view (hip-extended view) and the PennHIP radiographs during each phase of the heat cycle.
Results showed that even though the female hormone levels fluctuated (as expected) the PennHIP DI (hip laxity) and the OFA-type scores did not change significantly throughout estrus.
In Short: The rise in hormone levels during the female heat cycle does not affect hip laxity as measured by PennHIP or hip score from the standard hip-extended radiograph.
However, hormones released during the birthing process (relaxin) and during lactation (prolactin) can increase hip laxity and presumably subjective hip evaluation so at this time PennHIP testing is not recommended during or shortly after pregnancy. PennHIP recommends waiting 8 weeks post lactation or 16 weeks post whelping, before a PennHIP evaluation should be performed.