If you are anaemic, your blood is low in red blood cells, which carry oxygen. This means that your heart, which has to work hard to cope with the pregnancy anyway, has to work even harder to pump blood around the body even faster. It does this so that enough oxygen is delivered to the major organs and placenta. This explains some of the common symptoms of anaemia: racing heart and palpitations. Because your heart is pumping away so hard, it requires more energy than it would normally do; another reason why you may feel drained and weak.