There might be a deficiency in your prenatal vitamin (ie it's quality).
Or your diet may be insuficient.
Some good things to keep in mind about prenatals
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some high quality vits
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many others
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you can google individual brands for details.
Some things I reccomend during pregnancy are vitamin e,
fish oil & other essential oils(pharmaceutical grade only),
and pycnogenol(antioxidant).
Along with a good daily diet, liberal doses of cream of wheat can dramatically increase your energy levels. It dosen't have to be cream of wheat, any decent complex carbohydrate source can help provide more energy.
Also consider picking up a decent enzyme capsule to take with food. They signifigantly help with absorption.