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When to Schedule an Ultrasound

There are three important times during pregnancy that a sonogram is done:

  • Early in pregnancy before 8-10 weeks to confirm the fetal heartbeat, whether there are multiples, risks for a miscarriage, and if the pregnancy is located inside the uterus (as opposed to an ectopic pregnancy)
  • Around 11-13 weeks as part of the "First Trimester Screen"  which combines a sonogram and measurement of the fetus' nuchal fold with a maternal blood screening test of two pregnancy-specific substances (pregnancy-associated plasma protein-A and human chorionic gonadotropin (HCG)) to identify risk for specific chromosomal abnormalities, including Down Syndrome Trisomy-21 and Trisomy-18.
  • Around 18-20 weeks an anatomy sonogram to check for certain problems including fetal abnormalities.

Types of Ultrasounds

Most patients in the U.S. get at least a basic sonogram around 18-20 weeks, though it may surprise you to find out that presently most medical associations do not recommend a routine ultrasound during pregnancy. A level II ultrasound takes longer and is more extensive than a level I ultrasound, but what exactly is being done may change from one institution to the other.

Many doctors still debate whether it’s worthwhile for all pregnant patients to routinely have an ultrasound done. The question is not only during which time of the pregnancy it should be done, but there is also controversy what that ultrasound should be checking for and how successful routine ultrasounds are in the detection of fetal anomalies. An ultrasound in pregnancy can be either done as a “routine” ultrasound, an anatomy sonogram, usually between 18-20 weeks or it is done for specific reasons which usually depend at the time of pregnancy. There are essentially three different types of ultrasounds, depending on how much information is required

  • The limited ultrasound,
  •  The basic ultrasound
  • The targeted ultrasound.

Limited Obstetrical Ultrasound (Level 1 ultrasound)

This is performed to answer a specific, clinical question, when there is no time or need to do the more extensive basic sonogram. The Limited ultrasound often takes only a few minutes and can be done anytime during pregnancy. Early in pregnancy, it is often done to make sure the fetus is inside the uterus and to check the fetal heart rate, later on in pregnancy it is often done to check the growth of the fetus, to check the position of the placenta, and to look at the amount of the amniotic fluid.

Basic Obstetrical Ultrasound (Level 2 ultrasound)

This is more extensive than the limited ultrasound, and it takes normally between 30 and 60 minutes. It is done to answer a whole set of pre-defined questions and it also looks for specific fetal malformations, which in general the limited ultrasound does not do. This is usually the “routine” ultrasound most patients have at 18-20 weeks, and it is done at that time because the fetal anatomy is seen best, though it can also be done at other times too.

Targeted Obstetrical Ultrasound

This is usually done to look at specific fetal parts that were found abnormal on the basic ultrasound. Many ultrasound centers feel that there is not that much difference between a  basic ultrasound and a targeted ultrasound. There are no uniform opinions among specialist what exactly should be done on the basic obstetrical ultrasound. The procedure is often different from one ultrasound center to the other. In general, the following examinations are done with most

Basic Obstetrical ultrasounds:

  • Fetal Life (fetal heartbeat)
  • Fetal Movements
  • Number of Fetus
  • Presentation and Lie of Fetus
  • Amniotic Fluid Volume
  • Placental Location
  • Fetal Measurements (e.g. BPD, HC, FL, AC and other fetal parameters)
  • Fetal Weight
  • Fetal Anatomy (including the fetal head, face, ears, neck, chest, abdomen, and extremities, a basic 4 chamber view of the heart, the fetal spine, stomach, kidneys, bladder, umbilical cord insertion and number of vessels)
  • Evaluation of the uterus, cervix, and ovaries

Does an ultrasound show every issue/problem with the baby?

Don’t expect that every problem in the fetus is being detected. The ability to detect an abnormality in the fetus at an ultrasound exam depends on a number of factors, such as the size and position of the fetus, the amount of amniotic fluid around the fetus, mom's body type (whether she is overweight or slim), the type of equipment used, and, most importantly, the skill and experience of the person performing the sonogram. Some major problems (such as anencephaly) are quickly diagnosed, but others such as cleft palate are not.

How many abnormalities can be detected on the ultrasound?

Two studies, one was done in the US, the other in Europe showed an ultrasound detection rate of fetal malformations from 35% to 56%. In addition, there are both false-positive and false-negative examinations. A false-positive examination is an examination that shows something is abnormal, when in fact it is normal, and a false-negative examination shows a normal fetus when there is, in fact, an abnormality. When an ultrasound is being ordered during pregnancy always ask how extensive of an examination it is, who will be doing it, and make sure you know what the findings were.

Prenatal ultrasound cannot diagnose all malformations and problems of an unborn baby. Reported figures of detection of major malformations range from 40 to 90 percent, so one should never interpret a normal scan report as a guarantee that the baby will be completely normal. Some abnormalities are very difficult to find, and other abnormalities are unlikely to ever be seen on a prenatal ultrasound.

Although the terms ultrasound and sonogram are technically different, they are used interchangeably and reference the same exam. Ultrasounds are often done to look at fetal growth and to detect fetal and other anomalies. 

During pregnancy, an ultrasound can be done through the abdomen, with an abdominal probe, or transvaginally with a transvaginal probe.

An ultrasound scan is currently considered to be a safe, non-invasive, accurate and cost-effective investigation in the fetus. It has progressively become an indispensable obstetric tool and plays an important role in the care of every pregnant woman.