Your doctor will probably want to check your hemoglobin and red blood cell antibody measurements at this phase of your pregnancy. Though this will entail a blood draw, you can consider it preventive maintenance for a successful third trimester.
Anemia of Pregnancy
If any of your results indicate you are trending to be or already are anemic (below 10.5 g/dL), your doctor is likely to prescribe a separate iron supplement in addition to your prenatal vitamin. Iron supplements of this kind are usually a prescription you will pick up at your pharmacy, and you'll want to make sure you take them exactly as prescribed by your doctor. Unfortunately, increasing your iron uptake through a pill or dietary changes can result in constipation. To combat this potential, you will want to be sure to increase your daily water intake and try to get more fiber in your meals. These are steps you should be doing anyway, but they become more important if there is a chance you could become constipated. Eating spinach can add much-needed iron and roughage to keep your digestive tract moving as it should. Raisins and dried apricots are iron-rich and high in fiber, too, making them easy to transport small snacks on the go.
Red Blood Cell Antibodies and Rhogam
Testing your blood for red cell antibodies is also done about this time. If you are one of the rare Rhesus (Rh) negative folks, you could become sensitized to the blood from your potentially Rhesus-positive baby. To avoid this potential outcome, pregnant mothers are often given an anti-D (Rhogam) injection at 28 weeks and again after delivery to avoid complications in future pregnancies. If your Rh factor differs from that of your baby (you are negative, the baby is positive), it can result in your body creating antibodies against your baby's blood. These antibodies can cause problems for the next baby, which can lead to anemia and distress for the baby in utero.
