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Pregnancy Health Center

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Welcome to Week 16
16 Week Old Fetus
Your Baby: The First Flutters

The fetus measures 4.3 to 4.6 inches (12 cm) from crown to rump and weighs about 2.8 ounces (80 g). Despite its growing body, your fetus has plenty of room to move around within the uterus. Its head is erect, its legs are now longer than its arms, and its limb movements are becoming more coordinated. Its fingers and toenails are constantly growing and are now well formed. If you're eager to find out the gender and you're due for an ultrasound, you may be in luck -- as long as the baby cooperates.

Your Body: All About Amnios

What is it? An amniocentesis, often called amnio, is a procedure in which a needle is inserted through a woman's abdomen into the uterus. The needle is used to remove a small amount of amniotic fluid from the amniotic sac surrounding the fetus. An ultrasound is used to guide the needle into an area of the uterus away from the fetus. Since the fluid and the fetus are formed from the same cells and have the same genetic makeup, the fluid undergoes genetic testing in a lab.

What is the purpose of the test? One of the most common uses of the amnio is to identify genetic or chromosome abnormalities in the baby such as Down syndrome. It can also detect other conditions, including:

  • Genetic disorders known to run in the family
  • Neural tube defects such as anencephaly and spina bifida
  • Rh disease
  • Fetal lung assessment (the last two are done during the last half of the pregnancy)

Who takes the test? Between 80% to 90% of all amnios are done when women are over 35 years of age or will turn 35 before their due date. It is also performed if the results of the multiple marker test are abnormal. Other reasons for an amnio include:

  • If a couple has had a child with a chromosomal abnormality (i.e., Down syndrome) or with a metabolic disorder (i.e., Hunter's syndrome)
  • If a couple has had a child or a close relative with a neural tube defect
  • When the mother is a carrier of an x-linked genetic disorder (i.e., hemophilia)
  • When both parents are carriers of an autosomal recessive inherited disorder, such as Tay Sachs disease or sickle cell anemia
  • If one parent has a condition passed on by autosomal dominant inheritance (i.e., Huntington's chorea)
  • If the woman has to deliver early and the baby's lungs have to be assessed

When is the test taken? If the amnio is taken for chromosome analysis or molecular DNA analysis, it is typically performed between 16 and 18 weeks of pregnancy. In some cases, however, the test may be done up to a month earlier. If it is to assess fetal lungs, it may be taken shortly before delivery.

How safe is it? The risk of an amnio causing a miscarriage ranges from 1 in 200 to 1 in 400. When it does happen, it is usually because of an infection in the uterus, the water breaking, or labor being induced prematurely. While the numbers are small, the test should be done only when the benefits outweigh the risks.

You should talk to your health care provider about whether to have an amnio based on your risk factors and indications.

Weekly Tip

Many pregnant women get their best sleep during the second trimester. After all, your breasts are no longer tender, your tummy is finally calm, and some aches and pains have lessened. So before you get heavy and uncomfortable in the next trimester, try to get some extra sleep now. Stay in bed late on the weekend, or crawl into bed early and read a few more pages than usual. Revel in repose. It may be a long time before you have this chance again.


Review Date: 7/8/2017

Reviewed By: John D. Jacobson, MD, Professor of Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda Center for Fertility, Loma Linda, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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