The effects of maternal age and parity on placental and fetal development in the mare.
Authors: Wilsher S, Allen W R
Journal: Equine veterinary journal
Summary
# Editorial Summary: Maternal Age, Parity and Placental Development in Mares Wilsher and Allen investigated how maternal age and parity affect placental structure and function in Thoroughbred mares, recognising that whilst gross placental morphology is well documented, little is known about degenerative lesions that compromise placental efficiency and fetal development. Using stereological techniques on 84 mares stratified by age and parity, the researchers measured microcotyledon surface density (a key indicator of fetomaternal contact area) and calculated placental efficiency by correlating foal birthweight to the microscopic area available for nutrient exchange. Aged multiparous mares demonstrated significantly reduced microcotyledon surface density—attributed to endometrial degeneration—yet unexpectedly, maiden mares also showed lower values than young multiparous animals despite having histologically normal endometria, suggesting a "priming effect" whereby the first pregnancy optimises placental development for subsequent gestations; this was confirmed by following 11 maiden mares through two successive pregnancies, all of which showed marked improvements in surface density and foal birthweight at second parity. The findings reveal an inverse relationship between fetomaternal contact area and placental efficiency, implying the placenta actively adjusts its exchange capacity according to available surface area. For practitioners, this has important implications: breeding decisions in older mares and the value of allowing maiden mares one preparatory pregnancy warrant consideration, whilst the work underscores the need for further research into the mechanisms driving these morphological and functional placental changes.
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Practical Takeaways
- •Older multiparous mares have reduced placental efficiency due to endometrial degeneration; monitor foal birthweight and placental development carefully in aged broodmares
- •First pregnancies prime the endometrium for improved placental development; expect better foal viability and birthweight in second and subsequent pregnancies
- •Placental efficiency varies with mare age and parity; consider reproductive management strategies accounting for age-related changes in placental function
Key Findings
- •Microcotyledon surface density (Sv) was lowest in aged multiparous mares due to endometrial degenerative changes
- •Primiparous mares showed significantly lower Sv values than young multiparous mares despite having virginal endometrium, indicating a 'priming' effect of first pregnancy
- •Maiden mares showed significant increases in Sv values and foal birthweight between first and second parity
- •Foal birthweight reflects balance between fetomaternal contact and placental efficiency, with increased contact correlating to reduced placental efficiency