The female heat cycle (estrus) and hip screening
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 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 hormone levels fluctuated (as expected) the PennHIP DI (hip laxity) and the OFA-type scores did not change significantly.
In Short: The rise in hormone levels during the female heat cycle does not affect hip laxity as measured by PennHIP. However, hormones released during the birthing process (relaxin) and during lactation (prolactin) however, can increase hip laxity and hip evaluation at this time is therefore not recommended. PennHIP recommends waiting 8 weeks post lactation or 16 weeks post welping, before a PennHIP evaluation.