Why does blood sugar matter?
The placenta makes a hormone called Human Placental Lactogen that increases in the third trimester/ the farther you get along in pregnancy. The purpose of this hormone is to increase passage of nutrients to your growing baby. Normally this is a good thing (babies benefit from increased transfer of nutrients) and mom's pancreas responds appropriately to this hormone by making more insulin to keep increased blood sugar levels in check. However, when gestational diabetes occurs, mom's pancreas can't keep up and her blood sugar levels are elevated.
Sugar (or glucose) crosses the placenta by "facilitated diffusion". I think of this like a slide. The steeper the slide (the higher mom's sugar level is) the more sugar crosses over to baby. Mom's insulin does not cross the placenta. So, babies make their own insulin to regulate the sugar that crosses the placenta. Insulin is a growth factor, so babies that have higher insulin levels tend to be bigger (have bigger heads, shoulders, and abdomens). This can make delivery more difficult. It can also be stressful for babies after delivery (because they no longer have the sugar load to keep up with their higher insulin levels). As a result, baby's blood sugar can drop, sometimes to dangerous levels after delivery.
If mom's blood sugar levels are maintained in the normal range during the third trimester, these stressors for both mom and baby can often be avoided.
How can I maintain appropriate blood sugar levels?
The first step to optimizing blood sugars is to have small frequent meals (rather than 3 big meals) and to be mindful about your intake of carbohydrates (both amount and type). It's also helpful to have an adequate amount of protein, fat, and fiber, with each of your small meals/snacks.
If you are found to have gestational diabetes (and even in some cases, pre-gestational diabetes), you will get a glucometer and test strips, so that you can check your blood sugars (before you eat anything in the morning and 2 hours after meals). You will also likely have extra nutrition counseling to help optimize your blood sugar control.
Exercise can also help. If you have a meal or snack that is higher in carbs, going out for a 20 or 30 min walk can help lower your blood sugar level. Maintaining regular moderate level exercise most days of the week in the second and third trimester is really healthy for both you and baby.
What does it mean to be mindful of carbohydrate intake?
Everyone is a little different in how our bodies respond to sugar/carbohydrates. Watching intake (having smaller amounts / less often) of simple sugars like juices, soda, cereal, cookies, pastries, processed foods, and even some yogurts, which can have a lot of hidden sugars can be helpful. Carbohydrates are important nutrients, but if you can choose whole grains, like oatmeal or lower glycemic index carbohydrates, it will help to maintain lower overall blood glucose levels.