How Your Baby Grows During the Eighteenth Week
The crown-to-rump length of your baby in week eighteen of pregnancy, is about 5 to 5 1/2 inches. She will weigh about 5 to 5 1/2 ounces and continues to add bulk to her body at a rapid pace. Mothers should make sure their diets provide everything the baby needs to optimal growth.

By 18 weeks a skilled ultrasound technician will be able to see more details of the heart, and in fact, may be able to detect certain herat defects or potential problems. All the valves and chambers of the heart should be fully formed by this point, so a doctor or midwife will want the heart examined if you choose to have an ultrasound during your pregnancy.

Your baby is now swallowing and drinking the amniotic fluid, proving the digestive tract is in fully working order. She will also move around her watery world exploring and learning. She can be observed feeling the uterus, her toes, the umbilical cord and, if she is a twin, her sibling.

What the Mother Feels During the Eighteenth Week

These gentle movements of the baby may now be felt by the expectant mother, though are not likely felt by others yet.

The uterus has grown to the size of a canteloupe and your total weight gain is probably around 10 to 15 pounds. This level of weight gain is healthy and totally normal. A pregnant mother should not panic when she sees the scale going up, but rather focus on eating whole foods, rich in nutrients. Pregnancy is not the time to diet - it is the time to make healthy choices, eating small, frequent meals.

The growing size of the uterus will put pressure on major blood vessels in the body when you lie flat on your back. For this reason, pregnant women are encouraged to sleep on their sides, and avoid exercises that require lying on their backs.

Resources:
The Pregnancy Book: Month-by-Month by Dr. Sears

Your Pregnancy Week by Week by Lesley Regan

The Complete Book of Pregnancy and Childbirth by Sheila Kitzinger