A: In the second half of pregnancy, it's hard to judge how far along you are. Some women grow larger babies and some carry them higher in their abdomen, making the pregnancy appear further along. If you've had a baby before, your abdominal muscles have stretched in the past and your uterus may hang forward, making you look larger than you did in a previous pregnancy.

Doctors use a number of criteria to determine gestational age. If you have regular menstrual cycles, one traditional method of dating the pregnancy is by calculating how many weeks have passed from the first day of your last menstrual period. Without regular cycles and careful calendar records, this method becomes less reliable.

Measuring your uterus early in the pregnancy with a pelvic exam also provides clues about the age of your pregnancy. A 12-week uterus is usually about the same size as a grapefruit, for example. Later in the pregnancy, measuring the distance on your belly from your pubic bone to the top of the uterus helps doctors track the growth of the baby. However, having fibroids or being overweight can make this estimation less accurate.

Ultrasound exams done during the first trimester show the size of fetus and are an excellent determinant of fetal age. Later ultrasounds will also measure the baby's size but are less precise at judging gestational age because there is more variation between one pregnancy and another. Babies are born having different birth-weights; similarly, there is variation in the size of fetuses.

For you to accurately assess your true gestational age, ask your doctor to explain what criteria he is using to assess dates. If your due date is based on reliable information obtained early in the pregnancy, you should accept the fact that this pregnancy feels different than you expected.