Fetal ultrasound

By (embryologist) and (fertility counselor).
Last Update: 07/17/2014

The main ultrasounds have to be taken in the 12th, 20th and 32nd weeks of gestation, each one of them performed differently, as their purposes are not the same.

Provided bellow is an index with the 6 points we are going to expand on in this article.

What is it?

Ultrasound is an image-based diagnostic technique in which ultrasounds are sent out through a sounding line, the ultrasounds (high-frequency sound waves) get inside the organ to be analyzed, the foetus in this context, and thanks to physical phenomena such as reflection, a good share of the ultrasounds are transformed into electric signals that the ultrasound scanning display shows by the form of images. On order to ease the pass of the ultrasounds through the skin an aqueous gel is spread.

The ultrasound is harmless, both for the mother and the unborn, and allows the parents and the specialist to observe the development of the foetus, also aiding in the diagnose of fetal malformations. We need to bear in mind that a regular ultrasound does not always mean that foetus is doing okay, there are anomalies and malformations that the ultrasound may not make visible.

Scanning the foetus

According the some research, 60% of fetal malformations and 75% of foetus affected by trisomy 21 (Down Syndrome) can be detected using ultrasounds. There are biochemical analysis (triple testing) that also help detecting these kind of malformations.

The technique of choice will be the 2D ultrasound, although more complex modalities are available, such the 3D and 4D ultrasounds (same as 3D, with the particularity of seeing real-time images of the baby).

How is it performed?

There are 2 ways of performing an ultrasound during pregnancy: the vagina and abdominal ultrasounds.

  • Vaginal ultrasound , to be performed during the first trimester of gestation and even it might be a bit more discomforting than the abdominal, it provides more defined images.
  • Abdominal ultrasound, to be performed during the second and third trimester of pregnancy. The gynecologist will ask the mother, in order to get sharper images, to undergo the ultrasound in a half full bladder. So, prior to go the consulting room, she will have to drink some water.

Once the woman acknowledges she is pregnant, she must visit her gynecologist to undergo at least three ultrasounds, taking place during the weeks 12, 20 and 32 of gestation. There might be different complications that force the gynecologist to do more ultrasound evaluations.

First trimester

The first ultrasound will be taken somewhen between the 6th ad 13th weeks of pregnancy. In this time, the gynecologist will confirm the pregnancy through the visualisation of the embryo sac (or sacs) and its localisation (intrauterine or extrauterine), because the mother might be facing an ectopic pregnancy.

The foetus will be measured, its heartbeat felt and the exact week of gestation will be pronounced, as well as the estimated delivery date.

In this ultrasound the existence of severe morphological anomalies is ruled out, the cranium and limbs are observed, checking if the hearth it's not placed on the left side of the body.

The measurement of the nuchal fold also takes place in this period. If associated with the dimensions of the embryo and the age of the mother, it can indicate the existence of some chromosomal or cardiac anomalies. To make a good measurement of the nuchal fold, the ultrasound has to be made within the 11th and 13th weeks.

Second trimester ultrasounds

Second trimester

The second ultrasound will be performed during the second semester of gestation, between week 16 and week 20.

A complete morphological exam will be carried out, studying the heart and systematically measuring the head, abdomen and femur to make the sure the foetus is developing well.

This is arguably the most important out of the three, as small morphological anomalies can be detected, allowing the gynecologist to recommend the mother to do an amniocentesis, a test to study the cariotipe of the foetus and check everything is alright.

The gender will be determined, although there are times wherein the position of the foetus makes it hard to tell whether it's male or female organs.

Third trimester

The third ultrasound is to be performed during the 32nd and 34th weeks of gestion.

In this ultrasound the fetal position will be determined, and the appearance of late morphological anomalies will be evaluated, as well as the position of the placenta. The umbilical cord does not have to be tying the neck, and the amount of amniotic fluid is measured.

The foetus is measured and now the gender can be distinguished. The testicles, for instance, have already descended.

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 Teresa Rubio Asensio
Teresa Rubio Asensio
BSc, MSc
Master's Degree in Medicine and Reproductive Genetics from the Miguel Hernández University of Elche (UHM). Teacher of different Clinical Embryology courses at the UHM. Member and writer of scientific contents at ASEBIR and ASPROIN. Embryologist specializing in Assisted Procreation at UR Virgen de la Vega. More information about Teresa Rubio Asensio
Adapted into english by:
 Sandra Fernández
Sandra Fernández
B.A., M.A.
Fertility Counselor
Bachelor of Arts in Translation and Interpreting (English, Spanish, Catalan, German) from the University of Valencia (UV) and Heriot-Watt University, Riccarton Campus (Edinburgh, UK). Postgraduate Course in Legal Translation from the University of Valencia. Specialist in Medical Translation, with several years of experience in the field of Assisted Reproduction. More information about Sandra Fernández

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