The first thing to suggest is to rest as much as you can. Not in long stretches, but cat-nap where possible, put your feet up for even a few minutes as often as you can to recharge your batteries. It is also a good idea to start (or continue) some form of exercise which helps to increase your cardiac output such as swimming, running or aqua aerobics because these increase your stamina. Finally, just get your doctor or midwife to check that you are not anaemic.