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The placenta grows in your womb when you get pregnant. It is shaped like a plate. It joins your circulation to your baby's circulation. That lets certain substances pass back and forth between the two of you:

• Nutrients and oxygen from you pass to your baby through the placenta
• Waste products from your baby pass back to you through the placenta.

It takes a while for the placenta to grow enough to do its job. In fact, it doesn't start working fully until you are about 14 weeks pregnant.

The placenta needs to get more and more blood from you as your baby grows. To make this happen, cells from your baby grow into the tiny blood vessels in your womb. This makes the blood vessels bigger. There are about 150 of these big blood vessels.¹ They are shaped like funnels. They bring blood to the placenta.

By the time you are about 20 weeks pregnant, these blood vessels are four times to six times wider than they were before. They can carry much more blood. They stop growing at this point.

In women who get preeclampsia, these blood vessels don't grow properly early on in pregnancy. But it can take weeks or months for this to lead to the illness we call preeclampsia.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.