What your new size means

By now, most women are really beginning to show. At about 20 weeks, your health care provider may begin measuring your uterus. By measuring from your pubic bone to the top of your uterus (fundus), the number of centimeters should just about equal your weeks of pregnancy. At 23 weeks, the average woman will measure about 23 centimeters. This is a good way to keep track of fetal growth, since the uterus grows along with the baby.

If a measurement is smaller or larger than expected, a closer check of your last menstrual period dates might show you are not as far along in pregnancy or are actually farther along than originally thought. Or there may be a surprise. Women carrying twins may measure four or more centimeters larger than their corresponding week of pregnancy! Your health care provider may want to use ultrasound if your measurements are in question.

With all this growth, you may begin to see pinkish stretch marks forming on the skin of your abdomen, thighs, and even your breasts. Stretch marks are generally caused by a rapid increase in weight, and the marks usually fade after pregnancy. The skin on your belly may itch as it grows. Along with these discomforts comes some relief for many women. As your uterus grows out of the pelvic cavity, you may feel less pressure on your bladder, and less of the need to urinate as frequently.

Since some months have more days than others, using a calendar month system for pregnancy can be confusing. So, rather than January, February, etc., pregnancy dating uses lunar months, the months of the moon, which are 28 days long. A full-term pregnancy, which lasts about 280 days, is actually 10 lunar months long!

Most health care professionals use a week system to calculate pregnancy dates. This can be tricky, because it begins before you are even pregnant - with the start of your last menstrual period. Conception typically occurs about two weeks into the pregnancy calendar! Here's an example: If your last menstrual period began on April 1, conception probably occurred around the middle of April. By April 28, you have completed four weeks of pregnancy, and are starting your fifth week.

To figure your approximate due date, try the following system:

1. First, determine the first day of your last menstrual period.
2. Next, count back three calendar months from that date.
3. Lastly, add one year and seven days to that date.

Remember that each pregnancy is different and dates may have to be adjusted for longer or shorter menstrual cycles. Fewer than 10 percent of women actually give birth on their calculated due dates.