The baby's head is just under half the length of his body. His limbs are still relatively short but the hands and feet can look quite big. Early basic trunk and limb movements are taking place but it's too early to feel them at this stage.
Hormones are influencing your unborn baby's development and the ovaries or testes now begin to form. The testes will gradually descend but their structural development won't be complete until your child hits puberty. The ovary will produce eggs (see You are 20 Weeks and 6 Days) but these will remain in the early stages of development.
A minute genital tube forms the external genitalia but each sex appears the same at this stage. This is not entirely surprising since the phallus is only 0.09 in (2.5 mm) long. Your baby's bladder and rectum have now separated. The kidneys will take some time to fully develop: two buds grow up from the bladder to the tissue that will become the kidneys, one on each side. These so-called uteric buds form the ureters-the tubes that transport urine from the kidney to the bladder. The ureteric bud must successfully fuse with the kidney tissue in the pelvis. As the ureteric buds expands upward, the early kidneys developing in the pelvis will move upward to lie in the abdomen.
The potential effects of taking antihistamines in pregnancy aren't known, so it's best to err on the side of caution and not take them. However, if your symptoms are very severe, see your doctor since there is one antihistamine available by prescription that can be taken during pregnancy.
Nonidentical twins are always in separate amniotic sacs with a placenta each. If your babies are identical (from one fertilized egg) they may share the same placenta and/or amniotic sac, with a single membrane called the chorion surrounding them. These are known as monochorionic twins and will require greater monitoring. The arrangement of the placenta and amniotic sacs can be analyzed on an ultrasound scan.
When twins share a placenta, a blood vessel directly connects them. This can cause one twin to receive too much blood, which can cause heart problems; the other twin will get too little blood and not grow at the correct rate. This is called twin-to-twin transfusion and happens in about 10-15 percent of monochorionic pregnancies. The imbalance can sometimes be corrected by draining amniotic fluid from around the twin with the greater blood supply, or by using laser treatment to seal off some blood vessels in the placenta. An early delivery may be necessary.
Excerpted from Pregnancy Day by Day.
Copyright ©2009 Dorling Kindersley Limited.
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