Pregnancy Information Fetal Development Ultrasound Testing

Fetal development is evaluated through ultrasound, amniocentesis and blood tests. These tests also measure the weight of the fetus and estimate delivery time. Ultrasound and blood tests are the most common fetal development tests. High-risk pregnancies or unusual ultrasound findings require an amniocentesis follow-up.

Fetal Development and Ultrasound

Ultrasound is used to evaluate fetal development and estimate delivery date. Ultrasound is very sensitive: It can detect embryos by 5 1/2 weeks. Fetal heartbeat can be detected on ultrasound by six weeks.

If the date of the last menstrual period is unknown, ultrasound can date fetal age and determine a delivery date. Ultrasound estimates are most effective if the ultrasound is performed early in the first trimester.

Seeing an ultrasound image of the fetus is an emotional event for parents: Nothing drives home impeding parenthood quite as much as a seeing their baby moving on an ultrasound. New ultrasound technologies now provide 3D imaging of the fetus.

Biophysical Profiles and Ultrasound

A biophysical profile uses ultrasound to evaluate fetal development in high-risk pregnancies. Fetal heart rate, tone, amniotic fluid volume and fetal activity levels are all observed during a half-hour ultrasound. Abnormalities in the ultrasound may require further testing, either through blood tests or amniocentesis.

Ultrasound and Fetus Weight

Ultrasound helps monitor the length and weight of the fetus, two important factors in fetal development. An accurate assessment of fetus age can be determined by a crown to rump measurement between week seven and thirteen. Ultrasound is also used to determine head measurements.

Fetus Weight Classifications

In addition to ultrasound, fetus weight can be estimated by measuring the mother’s belly. Fetus weight that falls into the low and high percentiles may indicate fetal development problems.

  • SGA (small for gestational age): Fetus weight falls into the 10th percentile. Low fetus weight may be caused by fetal development defects (such as Down syndrome), or exposure to toxins. Fetal alcohol syndrome is a common cause of low fetus weight.
  • LGA (large for gestational age): Fetus weight is in the 90th percentile. High fetus weight increases the risk of delivery problems due to fetal size. Birth trauma, perineal trauma, jaundice and bleeding may result from high fetus weight. The heavier the fetus weight, the higher the chance that a cesarean delivery will be necessary. Obesity and diabetes a re common causes of high fetus weight. Both conditions should be controlled as much as possible during pregnancy.

Blood Tests and Fetal Development

Regular blood tests are common during pregnancy. MSAFP blood tests measure levels of maternal serum alpha fetal protein in the mother’s blood. High protein levels indicate a higher risk of fetal development disorders, including neural tube defects. If MSAFP blood tests yield high results, amniocentesis may be recommended.

Triple Screening Blood Tests

Combining MSAFP blood tests with blood tests for hCG and estriol provides a particularly sensitive test for fetal development problems. Abnormal protein levels from the three blood tests suggest chromosomal defects, such as Down syndrome. As with MSAFP blood tests, abnormal results from “triple screening” are often followed by amniocentesis.

AmniocentesisAmniocentesis: Detecting Fetal Development Disorders

Amniocentesis is a more invasive fetal development test than blood tests or ultrasound. During amniocentesis, a needle inserted through the abdomen collects a sample of amniotic fluid from the uterus.

Amniocentesis detects hundreds of fetal chromosome disorders, including cystic fibrosis and Down syndrome. Amniocentesis results have an accuracy rate of 99 percent. If needed, amniocentesis usually occurs between weeks fifteen and eighteen of pregnancy.

When is Amniocentesis Recommended?

Amniocentesis is recommended if blood tests or ultrasound detect potential fetal development problems. Amniocentesis is also recommended for pregnancies after age 35, or if the fetus is at high risk of fetal development disorders.

Amniocentesis can determine whether a fetus has lungs mature enough to survive delivery, a vital question during premature labors. Amniocentesis also detects Rh sensitization, a health risk that can occur if the mother is negative for Rh factor and the fetus is positive.

Amniocentesis Risks

Amniocentesis can trigger fetal miscarriages; some women decline amniocentesis for this reason. The risk of miscarriage due to amniocentesis is low. In most cases, amniocentesis is safe. A small number of women may develop Rh sensitization after amniocentesis.


Baby Center. (updated 2005). Amniocentesis.

Beers, M. H.