Ultrasounds performed during pregnancy are a valid diagnostic tool to detect the presence of fetal or mother problems during pregnancy. They can detect some defects including congenital ones such as spina bifida, heart defects, Down syndrome (which is then confirmed through genetic tests), structural and other organ abnormalities.
However, ultrasound scans are not always accurate in detecting all problems or complications that may arise. Their detection it depends on the quality of the ultrasound, the position of the fetus, the skills of the sonographer and the stage of pregnancy. Even if the use of 3D and 4D ultrasound images is used, some problems or details cannot always be identified.
It may be that in the early stages of pregnancy the doctor is not able to identify the heartbeat or, later on, has difficulty in correctly determining the sex of the baby, or errs in the predictions of the birth weight. This can happen, for example, if the machinery is dated and inefficient.
Why do ultrasound scans during pregnancy?
First of all, let’s clarify why It is important during pregnancy to perform at least the main ultrasound scans recommended. There are 3 basic ultrasound scans in pregnancy: dating ultrasound performed in the first trimester, structural / morphological ultrasound, and third trimester ultrasound.
Based on the pregnancy and the state of health of the mother and baby, the doctor will decide whether to add more.
Ultrasound scans are essential for a number of reasons that we will outline below.
- confirmation of pregnancy, is the most obvious reason
- To calculate the date of birth and the dating of pregnancy: ultrasound scans are much more efficient in estimating the date of delivery than the calculation based on the last menstruation date of the woman. This does not imply that the date provided by the ultrasound is the exact date of delivery, we know that the range in which you can give birth (at term) is very wide.
- For check the progress of pregnancy: the doctor may request additional ultrasound scans to check the overall development of the baby
- To check the position of the child: Before giving birth, the doctor may request an ultrasound to confirm whether the baby is in the correct position for birth or is in the breech position. This information is essential to understand how to cope with childbirth.
- To check (at the beginning of pregnancy) whether it is a single pregnancy or a twin pregnancy, which requires a specific path.
- Check for an ectopic pregnancy. If your doctor suspects an ectopic pregnancy, an ultrasound will help rule it out.
- Diagnosis of any complications: problems related to the uterus, amniotic fluid, ovaries or placenta
What can ultrasound scans fail to detect?
There are some issues in pregnancy that cannot be thoroughly examined or verified by ultrasound alone. Here are some examples:
- Problems detecting fetal heartbeat: Ultrasound scans help detect fetal heartbeat. However, when pregnancy is in its early stages, most ultrasound scans do not provide accurate results. Abdominal ultrasound scans do not detect fetal heartbeat unless the pregnancy is eight weeks. For pregnancies less than 8 weeks, they recommend using transvaginal ultrasound because it is more accurate. The fetal doppler also does not detect the fetal heartbeat until the pregnancy is past 12 weeks.
- Difficulty diagnosing a suspected abortion: Since abdominal ultrasound and fetal doppler cannot detect a fetal heartbeat in the early stages of pregnancy, it is difficult to detect a suspected abortion in this way. A deemed or internal abortion is an abortion that does not give any kind of external symptoms or signals. The pregnancy is interrupted and the only way to ascertain this is by eocgraphy. In these cases when there are doubts about the evolution of pregnancy, doctors should request two ultrasound scans about a week apart, and monitoring of beta HCG values. A single ultrasound, therefore, may not provide accurate results.
- Detection of congenital defects: Ultrasound scans are accurate for the diagnosis of congenital defects in the second trimester. When ultrasound is done between the 16th and 20th week of gestation, there is a better chance of detecting fetal problems. In particular, morphological ultrasound allows to obtain many useful data on fetal well-being and health. Ultrasound scans performed by specialized doctors can detect 3 out of 4 congenital defects when ultrasound is performed in the second trimester. However, some defects are more difficult to identify than others. Children with heart defects, joint stiffness, deformities and limb defects are easier to detect than those with Down syndrome for example. Even the latest technology can miss some details. This is especially true when the pregnant mom is obese or is pregnant with twins, triplets or more. There are also cases where an ultrasound detected congenital problems during pregnancy when in reality there was nothing. This misdiagnosis can cause unnecessary anxiety and worry in future parents.
- Identification of the sex of the child: There are not rare cases in which the doctor, during a second trimester ultrasound, makes a mistake in determining the sex of the baby. For example, he could mistake a loop in the umbilical cord for a penis. The position of the baby, as well as the experience of the sonographer, play an important role in correctly identifying the sex of the baby.
- Determination of the weight of the baby at birth: Abdominal ultrasound scans done in the second and third trimesters can help determine how much a baby weighs in the belly. However, this is an estimate that offers no guarantees to parents. It often happens that these results are also wrong by 300-500 grams, if not more, than the reality.
If the ultrasound shows a problem?
If the ultrasound shows that the baby has a problem, the doctor will explain the situation to you and will likely recommend further follow-up tests including genetic tests such as amniocentesis or CVS.
An ultrasound alone cannot detect all the problems a baby might have. Having a normal result on an ultrasound does not guarantee 100% that the baby will not have a birth defect or chromosomal abnormalities.
March of Dimes
Women’s and Children’s Health Network