Pregnancy

Can ultrasound scans during pregnancy give all the answers we are looking for?

ultrasounds in pregnancy are an excellent diagnostic tool but they are not 100% effective. Although they can detect a myriad of problems during pregnancy, in some cases they are not sufficient to be sure of a specific condition.

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 defects such as spina bifida, heart defects, Down syndrome (which is then confirmed through genetic testing), structural and other organ abnormalities.

However, ultrasound scans are not always accurate in detecting all problems or complications that may arise. Their detection depends on the quality of the ultrasound, the position of the fetus, the skills of the sonographer and the stage of pregnancy. Even with the use of 3D and 4D ultrasound images , some problems or details cannot always be identified.

It may be that in the early stages of pregnancy the doctor is not able to detect the heartbeat or, later, has difficulty correctly determining the sex of the baby, or mispredicts the birth weight. This can happen, for example, if the machinery is dated and inefficient.

Why perform ultrasound during pregnancy?

First of all, let’s clarify why it is important to perform at least the main recommended ultrasound scans during pregnancy . There are 3 fundamental ultrasounds in pregnancy: the dating ultrasound which is performed in the first trimester, the structural/morphological ultrasound, and the third trimester ultrasound.

Based on the pregnancy and the state of health of the mother and the child, the doctor will decide whether to add more.

Ultrasounds are essential for a number of reasons which we will outline below.

  • confirmation of pregnancy, is the most obvious reason
  • To calculate the date of delivery and the dating of the pregnancy:  ultrasound scans are much more efficient in estimating the date of delivery than the calculation based on the date of the last menstruation 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 it is possible to give birth (at term) is very wide. 
  • To monitor the progress of your pregnancy: Your doctor may order additional ultrasound scans to check the overall development of your baby
  • To check the baby’s position : Before delivery, your doctor may order an ultrasound to confirm whether your baby is in the correct position for birth or is instead in the breech position . This information is essential to understand how to deal with childbirth.
  • To verify (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 scan will help rule it out.
  • Diagnosis of any complications: problems related to the uterus, amniotic fluid , ovaries or placenta

What can’t ultrasound scans detect?

There are some pregnancy issues that cannot be accurately examined or verified by an ultrasound alone. Here are some examples:

  • Problems detecting the fetal heartbeat: Ultrasound scans help detect the fetal heartbeat. However, when pregnancy is in its early stages, most ultrasound scans do not provide accurate results. Abdominal ultrasounds do not detect the fetal heartbeat unless the pregnancy is eight weeks . For pregnancies less than 8 weeks, they recommend using transvaginal ultrasound because it is more accurate. Fetal Doppler also doesn’t detect the fetal heartbeat until the pregnancy is past 12 weeks.
  • Difficulty diagnosing a retained abortion : Because abdominal ultrasounds and fetal Doppler cannot detect a fetal heartbeat early in pregnancy, it is difficult to detect a retained abortion in this way. A retained or internal abortion is an abortion that does not give any kind of external symptom or signal. The pregnancy is terminated and the only way to ascertain this is by eocgraphy. In these cases when there are doubts about the evolution of the pregnancy, the doctors must request two ultrasound scans about a week apart, and the monitoring of the beta HCG values . Therefore, a single ultrasound may not provide accurate results.
  • Detection of birth defects : Ultrasound scans are accurate for diagnosing birth defects, in the second trimester. When the 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 us to obtain many useful data on fetal well-being and health. Ultrasounds performed by specialized doctors are able to detect 3 out of 4 birth defects when the 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 has found congenital problems during pregnancy when in fact there were none. This misdiagnosis can cause future parents unnecessary anxiety and worry.
  • Identification of the baby’s sex : It is not uncommon for the doctor to make a mistake in determining the baby’s sex during a second trimester ultrasound. For example, he could mistake a loop of 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.
  • Determining baby’s birth weight: Abdominal ultrasounds done in the second and third trimesters can help determine how much a baby weighs in the womb. However, this is an estimate that offers no guarantees to parents. It often happens that these results are wrong by even 300-500 grams, if not more, compared to reality.

If the ultrasound shows a problem?

If the ultrasound shows that your baby has a problem, the doctor will explain the situation and probably recommend further tests including genetic tests such as amniocentesis or CVS.

An ultrasound alone cannot detect all the problems a child may have. Having a normal result on an ultrasound does not guarantee 100% that your baby will not have a birth defect or chromosomal abnormality.

Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.

Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.

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