What is an 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.
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When should I 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.
What are the different 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 about what that ultrasound should be checking for and how successful routine ultrasounds are in the detection of fetal anomalies. An ultrasound during 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.
What is a 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.
What is a 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.
What is a 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, most basic ultrasounds check for:
- Fetal life (fetal heartbeat)
- Fetal movements
- Number of fetuses
- Presentation 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 will be detected. The ability to detect an abnormality during 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 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.
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.
Fetal ultrasound calculators & graphs
- Interactive fetal measurement calculations
- Fetal Growth Calculator
- Abdominal Circumference Calculator
- Head Circumference Calculator
- Femur Length Calculator
- Biparietal Diameter Calculator
- Fetal Weight Calculator (BPD, AC)
- Fetal Ultrasound Percentiles Calculator
- Ultrasound Estimated Fetal Weight Calculator (20+ formulas)
- Introduction to Ultrasound During Pregnancy
Reasons for an ultrasound during pregnancy
- Diagnosis and Confirmation of Early Pregnancy
- Vaginal Bleeding in Early Pregnancy
- Determination of Gestational Age and Assessment of Fetal Size
- Anatomy: Diagnosis of fetal malformation
- Placental Localization
- Cervical Length
- Fetal Size
- Multiple Pregnancies
- Amniotic Fluid: Too Much (Hydramnios) and Too Little (Oligohydramnios)