As your baby's lungs develop in the chest, there is nothing to separate them from the stomach, liver, and bowel in what will later become her abdominal cavity.
In adults, the chest is separated from the abdomen by the muscular diaphragm. When we breathe, the diaphragm moves downward and the ribs expand outward. This process allows air to enter the lungs.
Your baby's diaphragm forms from four in-folds of tissue. First seen at around this week of pregnancy, these folds gradually expand inward, fusing together and closing the space by the end of this week. In the center of the diaphragm there are openings for the esophagus to the stomach, main artery to the body-the aorta, and main vein returning blood from the lower body, the inferior vena cava. As your pregnancy progresses, muscle fibers gradually strengthen your baby's diaphragm, which later allows her to make breathing movements.
If you're expecting twins or more, blood-based screening tests for Down syndrome can mislead, since they rely on measuring the amounts of circulating AFP (alfa-fetoprotein) and other markers, which are present in much higher levels when there's more than one baby. That's why your best bet for screening is the nuchal translucency test from 11-14 weeks.
It's useful to be aware of the screening tests and diagnostic tests that will be available in the weeks to come. Your doctor will talk through the pros and cons of having each test. Some abnormalities may be detected at the 18-22 week scan.
Excerpted from Pregnancy Day by Day.
Copyright © 2008 Dorling Kindersley Limited.
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