Pregnant bellies are measured in centimeters -- usually starting around 20 weeks -- and there's a simple formula for calculating how large your tummy should be at any given point in your pregnancy. Start with the number of weeks you're pregnant, then add two to that number and also subtract two from that number, which will give you the range your belly should be within. For example, if you're 30 weeks pregnant your belly should be between 28 and 32 centimeters. If you're 25 weeks along, your belly should measure between 23 and 27 centimeters.
There are a variety of reasons why your belly could be measuring large, like swelling or fluid retention (a common problem during pregnancy) or simply being a bigger woman before you became pregnant. The most serious cause of having a too-large tummy, however, is gestational diabetes. When a woman develops diabetes during pregnancy, her baby receives too much sugar, causing him to be bigger than he normally would. This can also cause excess amniotic fluid, which increases belly size too.
If your doctor is concerned about your belly, he or she will order an ultrasound to check on your baby. The ultrasound will measure the circumference around your baby's stomach and head as well as the length of her legs to see if there may be a problem. Your doctor will also likely check for gestational diabetes with a glucose tolerance test, if you haven't already had one.
Should you have gestational diabetes, your doctor will put you on a diet to cut out excess sugar and, if necessary, will prescribe medication. Although gestational diabetes can be serious if left unchecked, most women with the disease give birth to healthy babies, and after that the condition usually goes away.
Lastly, know that friends and family (and even total strangers) love to discuss the size of a pregnant woman's belly. If you're upset by anyone's comments on yours, try to ignore them. It's your doctor's job to monitor your pregnancy and unless she's concerned about your belly size, no one else should be.