Mild exercise is key to preventing swelling in a heavily pregnant mareSome mares in late pregnancy develop a lot of swelling around the udder, which may even extend forward along the flanks. Occasionally, the mammary vein along the lower abdomen becomes swollen as well, almost as far forward as the girth area.
This swelling-or edema-is an accumulation of fluid in the tissue spaces, caused by a disruption of the normal interchange of fluid between the capillaries, tissue spaces, and lymph vessels. The fluid is not able to move freely between the blood vessels and lymph system and tends to accumulate in the tissue spaces.
In the heavily pregnant mare, a large, distended, and heavy uterus may put pressure on the mammary veins, compressing them so there is poor blood return. The backup effect also puts more pressure on the capillaries and fluid tends to leak out. Inactivity and subsequent poor circulation accentuates the problem.

Vicious circle
As the mare gets closer to term and heavier with foal, she becomes more clumsy and sluggish and tends to move around less, which in turn aggravates the circulation slow-down problem. It becomes a circular situation-the heavier she gets, the less she exercises, the more swelling she develops, and the less she feels like exercising. She may even have trouble getting up and down. Some mares rarely lie down during the last few weeks or days of pregnancy, because the effort of getting back up is so great.
A mare at pasture will generally move around some during the normal activities of grazing and going to water, and her edema may not get extreme. But a mare confined in a stall or paddock may not move around at all, especially as she becomes uncomfortably heavy in late gestation. The inactive pregnant mare may also "stock up," her legs filling with fluid-also due to sluggish circulation and capillary leakage.

Exercise important
To prevent or alleviate this type of edema, the mare needs mild daily exercise, which will help her circulatory system function better, clear some of the excess fluid from the tissues, and enable her to feel better (and if she feels better, she may do more moving around on her own instead of spending most of her time standing). The heavily pregnant mare should not have vigorous exercise; she is uncomfortable and clumsy, and you do not want to cause her to stumble or fall. But brisk walking can be very helpful to her condition and well-being.
If she is confined and not moving around much, try to give her a good walk at least twice daily, morning and evening. Leading her on a brisk walk for 20-to-30 minutes at a time will help relieve the swelling or prevent more edema, as well as help keep her muscle tone in better readiness for delivering her foal. The mare who gets some regular exercise generally has less trouble foaling than the totally unfit, inactive mare who has spent the last part of gestation just standing around.

The fetal envelope
THE unborn foal is entirely surrounded by enveloping membranes, which include the placenta, and inside of that, the amnion. The placenta attaches directly to the lining of the uterus (and the uterine blood vessels supply the placenta and developing fetus) except at the cervix. This unattached spot is called the cervical star.
During labor, while the fetus is positioning itself for birth, the cervix relaxes and the uterus begins its rhythmic contractions, putting pressure on the fluid contents that surround the fetus in its "envelope." The only direction the fetus can go is through the cervix.
Pressure from the uterine contractions forces the contents through the relaxing cervix. The placenta ruptures at that spot, and this allows the inside envelope-the amnion-and the fetus inside it to start through the birth canal. The foal is usually still encased in the amnion sac as he emerges, but this membrane breaks during the struggle of birth, or soon after.

Protective nourishment
The fluids that were between the inner and outer envelopes usually precede the birth of the foal (breaking of the water). The water bag is a separate membrane within the placenta, separated from the amnion sac by the amber-colored fluid that then escapes when the mare's water breaks. These envelopes and their fluids cushion and protect the growing fetus and serve a number of other functions, such as nourishing the developing embryo during the early stages of pregnancy.
The placenta (the thick outer envelope) begins detaching from the uterine lining during labor, finishes detaching soon after the birth, and follows the foal out of the birth canal about 15-to-45 minutes later, except in abnormal situations where the placenta is retained within the mare.