be aware of your body's vitamin levels during pregnancy and while breastfeeding...

if you are going to breastfeed, it would be ideal for you to get a blood test of, at the very least, your vitamin levels.   try to get this test done in the baby's 4-8-months-old zone.  by this time, you can accurately detect what the baby is sucking out of you (ie making you deficient in) in time to change it without it affecting baby's health.   lactating women are commonly deficient in vitamin d, b12, and zinc.

regarding pregnancy, it is often normal for your ob/gyn to conduct a blood test near the beginning of your pregnancy to test for anemia, etc.  of course, you can certainly request a more in depth test too.

while pregnant and breastfeeding, your recommended dietary allowance of these most commonly deficient vitamins is likely:

vit d3 (as calciferol) = 5,000 IU -- or spend 20 minutes/daily in sunlight, no sunscreen.
vit b12                     = 2.8 mcg
zinc                         = 12 mg

for pregnancy and lactating vitamin and mineral supplement fact sheets, check out the national institute of health's site.  although, i know, for example that they really low-ball on vitamin d.  read more about 5,000 iu of vitamin d here.

the best way to know for sure what you should take to supplement is to get a blood test--if you are deficient in something, you may need up to 5 times more than the rda.