The effects of maternal age and parity on placental and fetal development in the mare.
BACKGROUND
The normality of equine placentation is essential for fetal health and development. Substantial information exists on the gross morphological status of the placenta but few studies have addressed the problem of degenerative lesions that interfere with placental morphology and placental efficiency.


OBJECTIVE
Degenerative changes in the endometrium with increasing age and parity are reflected in the morphology and density of the placental microcotyledons.


OBJECTIVE
To assess placental efficiency on the basis of foal birthweight as a function of total microscopic area of fetomaternal contact.


METHODS
Stereology was used to examine the placentae of 84 Thoroughbred mares grouped on the basis of age and parity. Placental efficiency, assessed by expressing foal birthweight as a function of total microscopic area of fetomaternal contact, was also determined.


RESULTS
Mare age and parity influenced the development of the microcotyledons and microcotyledon surface density (Sv) was lowest in aged multiparous mares, presumably due to degenerative changes in their endometrium. However, primiparous mares also showed significantly lower Sv values than young multiparous mares, despite the virginal endometrium of the former group. This apparent 'priming' effect of a first pregnancy on microcotyledonary Sv was illustrated further by 11 maiden mares followed in 2 successive pregnancies. They all showed significant increases in Sv values in their second parity, with equivalent improvements in foal birthweight.


CONCLUSIONS
Foal birthweight is a reflection of the balance between fetomaternal contact and placental efficiency. Increases in fetomaternal contact are correlated to reductions in placental efficiency, which may reflect the ability of the placenta to modify its exchange capabilities.


CONCLUSIONS
Further work is needed to elucidate how such changes in microcotyledon morphology and efficiency are brought about.
Wilsher
S
S
University of Cambridge, Department of Clinical Veterinary Medicine Equine Fertility Unit, Mertoun Paddocks, Woodditton Road, Newmarket, Suffolk CB8 9BH, UK.
Allen
W R
WR
D000367:Age Factors; D000375:Aging / Q000502:physiology*; D000818:Animals; D001724:Birth Weight; D005314:Embryonic and Fetal Development / Q000502:physiology*; D005260:Female; D006736:Horses / Q000196:embryology / Q000502:physiology*; D008423:Maternal Age; D010298:Parity; D010920:Placenta / Q000502:physiology; D010929:Placentation; D011247:Pregnancy; D011270:Pregnancy, Animal / Q000502:physiology*