What is foetal ultrasound in the first, second and third trimester?

Ultrasound is a diagnostic imaging technique based on the emission of ultrasound (high frequency sound waves) by a probe.

Through this diagnostic test it is possible to obtain images of the fetus, placenta and amniotic fluid. There are several methods for performing fetal ultrasound: abdominal and transvaginal.

An advantage of ultrasound is that it does not use X-rays, making it a safe technique for both mother and baby. In addition, the first gestational ultrasound may be performed at 5-7 weeks of pregnancy for confirmation.

The different sections of this article have been assembled into the following table of contents.

What is an ultrasound?

The ultrasound penetrates the organ to be studied, in this case the fetus, and through various physical phenomena such as reflection, part of the ultrasound is transformed into electrical signals that appear on the ultrasound screen in the form of images. An aqueous gel is used to facilitate the passage of ultrasound through the skin.

The main ultrasounds are performed at 12, 20 and 32 weeks of pregnancy. Each of them is carried out in a different way since their purpose is different. Additional ultrasound scans may be performed in specific cases where there are possible complications that require the gynecologist to perform additional ultrasound examinations.

Risks of fetal ultrasound

Ultrasound is not a harmful technique for either the mother or the fetus, but quite the opposite, since:

• It helps to observe the development of the fetus inside the mother's uterus.
• It facilitates the diagnosis of malformations in the fetus.

Advantages of ultrasound in pregnancy

Despite these benefits, it is important to be clear that a normal ultrasound does not always mean that the fetus is normal. There are anomalies and malformations in the fetus that are not visible by ultrasound.

According to several scientific studies, ultrasound can detect 60% of fetal malformations and 75% of fetuses affected by trisomy of chromosome 21 (Down syndrome). There are biochemical analyses (triple test) that also help to detect this type of malformation and corroborate the diagnosis.

What types of ultrasound scans are there?

Fetal ultrasounds can be performed in two ways. On the one hand, it is possible to perform a transvaginal ultrasound by introducing an ultrasound scanner through the vagina. This type of test is more common during the first months of pregnancy.

On the other hand, fetal ultrasound can be performed transabdominally. In this case, only the ultrasound scanner should be slid over the abdominal area. Generally, transabdominal ultrasound is performed in the second and third trimester of pregnancy and the pregnant woman is advised to come with a full bladder so that the image is clearer.

Type of ultrasound in pregnancy

Apart from taking into account the way the ultrasound is performed, it is also possible to distinguish different types of ultrasound depending on the type of image obtained:

• 2D ultrasound: this is the traditional fetal ultrasound that allows to observe the fetus in two dimensions. In addition, the images are in black and white. This type of ultrasound provides information about the growth and development of the fetus.
• 3D ultrasound: fetal volume and color are added to the classic 2D image. For this reason, 3D ultrasound allows a clearer and sharper image of the fetal structures to be obtained.
• 4D ultrasound: in addition to seeing the fetus in three dimensions, this type of ultrasound offers a fourth dimension: the movement of the fetus in real time. This helps to learn about the baby's motor skills, as well as its general behavior and response to stimuli.
• 5D ultrasound: thanks to advances in technology, the 5D ultrasound image is much more detailed and realistic than other types of ultrasound.

Differences in imaging between 2D, 3D and 4D ultrasound

In any case, all types of ultrasounds have the same purpose, that is, to observe the baby. The choice of one type of ultrasound or another will depend on the situation of each pregnant woman, as well as her economic situation. For example, 2D ultrasound is much cheaper than a 5D ultrasound, but the images will not be as accurate.

Ultrasounds in pregnancy by trimester

Generally, and taking into account the protocols of the Spanish Society of Obstetrics and Gynecology (SEGO), the number of fetal ultrasounds recommended throughout pregnancy is at least 3. Specifically, it is usual to perform an ultrasound scan in each trimester of gestation.

On the other hand, if the pregnancy is considered risky, the specialist may advise to perform an ultrasound every month or every two months in order to have a better control of the gestational situation.

The following are the characteristics that are evaluated in each of these ultrasound scans.

First trimester

The first ultrasound of pregnancy is performed between the 6th and 13th week of gestation. The objective of fetal ultrasound in the first trimester of pregnancy is:

• Confirm pregnancy by visualization of the embryo sac(s).
• Determine the location (intrauterine or extrauterine) of the fetus, as it could be an ectopic pregnancy.
• Take measurements of the fetus.
• Listen to the fetal heartbeat.
• Determine the exact week of gestation and thus be able to establish the approximate date of delivery.

In this ultrasound it is verified that there are no major morphological abnormalities, the skull and all limbs are observed including the heart is located on the left side of the body.

When is the first trimester ultrasound done?

Another important measurement performed with the first ultrasound of pregnancy is the nuchal fold measurement. This determination of the nuchal fold together with the size of the embryo and the age of the mother could reveal certain chromosomal or cardiac abnormalities.

Second trimester

When the third month of pregnancy has ended, the next fetal ultrasound should be performed between 16 and 20 weeks of gestation. Therefore, this ultrasound would be performed in the second trimester of pregnancy and will be performed there:

• Complete morphological examination.
• Study of the heart.
• Measurement of the head, abdomen and femur to check the correct growth of the fetus.
• Determination of the sex of the baby, except in cases where the position of the fetus prevents a correct view of its sexual organs.

Ultrasounds during the second trimester of pregnancy

This ultrasound is perhaps the most important of the three pregnancy ultrasounds, since it can detect small morphological abnormalities that may cause the gynecologist to advise the mother to undergo an amniocentesis. In addition, most commonly, ultrasound in the second trimester of pregnancy is performed abdominally.

Third trimester

The third fetal ultrasound is performed between 32 and 34 weeks of gestation, at this time, the ultrasound will allow:

• Determine the fetal position.
• Take measurements of the fetus.
• Clearly see the sex of the future baby, since at this time the testicles of boys have already descended.
• To discover the appearance of late morphological anomalies.
• Assess the position of the placenta.
• Observe that the umbilical cord does not encircle the neck of the fetus.
• Measure the amount of amniotic fluid.

Fetal ultrasound in the third trimester of pregnancy

In the case of a twin pregnancy, the third trimester ultrasound will show the status of each baby in the uterus. In addition, this test will allow the measurement of cervical length and, therefore, it will be possible to predict whether there is a possibility of preterm labor.

FAQs from users

When is the first pregnancy ultrasound after IVF?

It is usual to perform the first ultrasound scan in the 6th-7th week of pregnancy to confirm the pregnancy and to observe that the pregnancy is progressing correctly. Therefore, the first ultrasound test takes place about 3-5 weeks after the embryo transfer.

In this first ultrasound after IVF, the gestational sac, the yolk vesicle, the heartbeat and the embryonic bud will be observed.

Is 3D ultrasound safe for the foetus?

Yes, 3D ultrasound is not a dangerous imaging test for the foetus, it is completely safe. Moreover, this test does not pose any danger to the mother's health either.

This is because 3D ultrasound does not emit radiation, but its methodology is based on the application of ultrasound.

How long is the first trimester ultrasound scan?

The general duration of the first ultrasound of pregnancy is approximately 30 minutes, although it will depend on the position of the foetus.

If the foetus is not in a suitable position, it will make it difficult to see the foetus and measure the nuchal translucency, for example

How many ultrasound scans are necessary during pregnancy?

This will depend on whether the pregnancy is risky or not. For women with a normal pregnancy, it is usual to do one ultrasound scan per trimester. Each one will provide different information, but it is important that these ultrasounds are done in the right week of pregnancy.

On the other hand, if the pregnancy is at risk, some specialists recommend a monthly or bimonthly ultrasound. In this way, the pregnancy is perfectly controlled so that complications do not occur.

Suggested for you

If you would like to learn more about other tests performed during pregnancy, we recommend you visit this article: Prenatal pregnancy control: tests, analyses and ultrasound scans.

On the other hand, ultrasound in the second trimester of pregnancy is quite useful since it will inform of possible anomalies in the fetus that require a more precise diagnosis. Therefore, the specialist may recommend an invasive prenatal diagnostic technique such as amniocentesis.

If you are interested in this topic and want more detailed information, we recommend you to access this link: Amniocentesis: what are its indications and possible risks?

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