1 Mercury exposure at high levels can harm the brain, heart, kidneys, lungs, and immune system. 2  High levels of methylmercury in the bloodstream of unborn babies and young children may harm the developing nervous system, making the child less able to think and learn. A highly toxic form (methylmercury) builds up in fish, shellfish and animals that eat fish. 2  Fish and shellfish are the main sources of methylmercury exposure to humans.
The length of a normal pregnancy is usually 335-342 days, but occasionally can range from as little as 315 to over 400 days. About a month before foaling many mares start to develop edema low down on their abdomen.
A veterinarian with an ultra-sound machine can check a mare as early as 18 to 20 days after conception, if they are experienced at it. We have tended to wait until the mare has missed one heat cycle before having one done, just in case.