Fetal Development Weeks 18 Through 21

See what's happening with your baby in the fifth month of your pregnancy.

Week 18

The unique swirls and whorls that are your baby's fingerprints begin to appear at the tips of her fingers this week. Her hands begin to make grasping motions, and she might even start sucking her thumb. This self-soothing habit can stick around into a child's toddlerhood and beyond, but all kids give it up eventually.

The placenta, that all-important nurturing organ, is now almost as big as the baby herself. Besides providing her with a steady diet of nutrients and oxygen, this dark blobby mass also receives waste products from the fetus's blood and deposits them in the mother's blood. The waste is then filtered out by the mother's kidneys.

Next: Week 19

Week 19

Welcome to the fifth month of pregnancy! Here's proof that life is an endless cycle: less than half a year ago your baby was just an egg herself, but by now, if she's a girl, she's got eggs of her own. As of this week, a female fetus's ovaries have already developed the tiny follicles that hold all the eggs she'll ever have -- as many as six million. Some of her ovarian follicles may even develop and grow, thanks to those supercharged female hormones flowing from mother to baby. But these tiny eggs, or ova, never develop to the point of ovulation. That won't happen for years and years yet- not until your little girl reaches puberty.

Throughout baby's body, nerves are being coated with a fatty substance called myelin, which insulates the nerves so that impulses can flow smoothly. The touch of a soft blanket, the sound of your voice: all of these things will be carried by these speedy paths from sensory organs to brain.

Next: Week 20

Week 20

Send up a cheer, pop a bottle of sparkling grape juice- you and your baby have reached the halfway point of pregnancy. What follows is a time of dramatic growth for your little one, who, on average, now measures up to 8 inches long from head to rump, and weighs in at a whopping one pound. He's come a long way since his days as a microscopic speck, but he's still got a lot of growing to do: at birth, the average newborn is 18 to 21 inches long and weighs 6 to 9 pounds.

The fetal heartbeat is growing stronger now. In the latter part of the second trimester, its distinctive whoosh whoosh can be detected with a simple at-home monitoring device (available at some drugstores). The baby's heart beats rapidly -- about twice as fast as his mother's heart.

Next: Week 21

Week 21

What does a baby dream about in the womb? This intriguing question arises in the coming weeks as your baby's sleep patterns begin to emerge (though, according to most research, dreaming may not begin until around week 28). When is your little one snoozing? You might be able to tell if her movements show a marked slow-down at certain times of the day. Some scientists suggest that your eating and sleeping habits and the levels of light and noise around you function as signals and help baby set her internal clock to match the outside world's even before birth.

At this point in its development, the baby's brain begins to grow very quickly, especially in what's called the germinal matrix. This structure deep in the middle of the brain serves as a kind of factory for brain cells and disappears shortly before birth. But the brain's amazing expansion program continues, as its capacity constantly grows until the age of five years.

Reviewed 2008

All content on this Web site, including medical opinion and any other health-related information, is for informational purposes only and should not be considered to be a specific diagnosis or treatment plan for any individual situation. Use of this site and the information contained herein does not create a doctor-patient relationship. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.

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