At week 16 the baby is about 4 inches long and about 4 ounces in weight. The eye has developed enough at week 16 that light is able to be perceived by the baby and it is even possible to see some eye movement at this time.
Week 16 is when we can do the optional blood test called second trimester screening. This has gone by various names in the pat as the test becomes more advanced. It has been called the “AFP” test, the “triple screen”, and the ‘quad marker”. These all refer to the same second trimester screening test. The blood test will not tell you if the baby has a genetic problem but rather if the chances of having a problem are increased. This can test for the risk of downs syndrome, certain trisomies like trisomy 18, and neural tube defects. Neural tube defects are problems with the brain stem and spinal cord. The cutoff for considering the test abnormal is 1:190. This means if there is 1 chance out of 190 that the baby has the problem, the test is considered abnormal. As you can see, most of the “abnormal” tests have normal babies.
If the second trimester screening test is abnormal, the patient is referred for amniocentesis. This involves placing a needle through the women’s abdomen and obtaining amniotic fluid from around the baby. This test will tell for sure if one of these abnormalities is present but it has a 1:200 chance of causing a miscarriage. None of the abnormalities that are tested for are curable or treatable while the baby is in the womb. Because of this many women decide not to do the genetic testing. The testing is optional and there is no right or wrong answer about whether or not to have it done.
Women that are 35 or older are considered “advanced maternal age”. Do not worry if you are in this age group your risks of a problem are very low. The risk of a baby with a genetic abnormality in women 35 years old is 1:190, because this risk is higher than the risk of the amniocentesis; women that are 35 or older are offered an amniocentesis without doing the blood test. That does not mean you have to do the amniocentesis or even that you necessarily should do the test. It means that the test is available for you if you desire to do it.