The effects of intrafetal ACTH administration on the outcome of pregnancy in the mare.

This research study investigates the effects of injected ACTH into fetal horses on the length of pregnancy and the likelihood of fetal survival. The findings reveal that ACTH injections can lead to earlier birth and well-developed foals, though not without some potential complications.

Study Design

• The study involved 23 pony mares who were at least 300 days into their pregnancy.
• The mares were divided into three groups. The seven mares in the control group (Group 1) received three consecutive injections of sterile water into their fetuses. The 16 remaining mares received ACTH injections into their fetuses. Of those 16, four were allowed to give birth naturally (Group 2), and seven were induced to give birth within three days of the last injection (Group 3).

Findings

• The Group 1 mares all successfully gave birth to live foals, with a typical pregnancy length of 327 days. One control mare did, however, experience a miscarriage 16 days after the final injection.
• The mares in Groups 2 and 3 showed significantly higher maternal plasma progestagen concentrations following the ACTH injection than those in Group 1.
• Group 2 mares gave birth earlier than controls, at a median of 314 days. Foals born to Group 2 mares showed regular postnatal adaptive responses and hormonal patterns comparable to those of Group 1 foals.
• Group 3 mares gave birth at about 306 days gestation. Unlike the foals from Groups 1 and 2, these foals were less mature.
• Mares that received ACTH injections had some complications. Five mares had a miscarriage within 48 hours of the ACTH injection. Eight of the ACTH-injected fetuses were meconium-stained at delivery, and eight mares had placentas that were abnormally thick, including three that had miscarriages.

Conclusions

• The data shows that ACTH injections into fetuses can lead to earlier birth and well-developed foals, but it also increases the risk of complications such as miscarriage and abnormal thickening of the placenta.
• The researchers theorize that the ACTH injections likely promote mature fetal development and reduce the length of pregnancy by triggering adrenal regulation of fetal maturation and production of maternal progestagens.