Three in number, obstetric ultrasounds are a highlight of pregnancy monitoring. A key examination in prenatal screening, fetal ultrasound can be used to date the pregnancy, control the well-being of the baby’s growth, and detect any chromosomal abnormalities or morphological malformations.

Ultrasound is a medical imaging technique based on ultrasound, that is to say sounds at high frequencies (between 2 and 10 MHz), inaudible to the human ear. The ultrasound probe emits ultrasound which propagates through the body. When they encounter an obstacle (bone, skin, organ…), it sends back an echo – hence the term ultrasound. This echo is converted into an electrical signal then into an image on a monitor, in different shades of gray depending on the material crossed. The waves pass quickly through liquids, which then appear black on the screen. In gray, these are the organs which partially reflect the waves. Finally in white, hard structures like bones, which block waves.

Ultrasound therefore makes it possible to visualize the inside of the body. In this case for fetal ultrasounds, the interior of the belly of the future mother but also the interior of the fetus.

These images make it possible to visualize the baby in two dimensions, in a “section” image. In fetal ultrasound, different types of cuts are used:

• the transverse (or axial) section makes it possible to observe the different “stages” of the baby and thus to control the morphology of the organs and to carry out certain measurements or “biometrics”;
• the frontal cut makes it possible to observe the face of the fetus;
• the sagitalle cut is a profile cut. It is also used for biometrics, for example to measure the bone of the femur.

3D ultrasound is also based on ultrasound, but unlike the conventional probe which is flat, the 3D probe is rounded and contains a specific device. This probe makes it possible to simultaneously carry out several contiguous 2D sections. Assembled by specific software, these sections make it possible to obtain a 3D image of the baby, that is to say in volume.

As to 4D echo, the “4th dimension” refers to time. 4D ultrasound is therefore a 3D ultrasound in real time and in motion.

Le doppler also uses ultrasound to visualize the movements of liquid. It works like sonar: when they are reflected off a moving surface (the baby’s heart, the umbilical artery), the frequency of the sound waves varies. This variation gives a sound which is transcribed on a graph. The Doppler thus makes it possible to measure the blood flow in different veins and arteries of the baby and the mother.

Performed as part of obstetric follow-up, ultrasound is not dangerous for the baby. The WHO and the World Federation of Ultrasound Societies have concluded that there is no concrete evidence that could demonstrate any harmful effect of ultrasound under the usual conditions of obstetrical diagnosis.

On the other hand, be careful with commercial ultrasounds performed to keep a memory of the pregnancy. “In a medical ultrasound, the ultrasound beam is constantly shifted and ultrasound exposure to every area of ​​the fetus is brief. This is not the case in a commercial ultrasound where it is necessary to continuously expose localized parts of the fetus to ultrasound, especially the skull and genitals. The risks of prolonged exposure to ultrasound are not zero, especially on the brain and the eye of the fetus, especially in the first trimester, or if the patient has a temperature, ”warns the French National College of Obstetricians ( CNGOF) in a press release (1).

For each pregnancy ultrasound, there are precise specifications established by the Prenatal Screening Ultrasound Technical Committee and reviewed in 2016 by the National Conference on Obstetric and Fetal Ultrasound (CNEOF). This standard exists for screening ultrasounds for classic pregnancy monitoring and for so-called diagnostic ultrasounds (DDD ultrasounds) performed when a problem is suspected. These learned societies have also established fetal growth charts.

During the examination, the ultrasound machine therefore checks the organs one by one, measures specific parts of the baby’s body, checks the appendages (placenta, umbilical cord, amniotic fluid) according to strictly defined quality criteria.

The results are transcribed on a report to keep in her pregnancy file, one of the copies of which is given to the gynecologist or midwife who ensures the follow-up of the pregnant woman.

On this report, many abbreviations appear. Here are the main ones:

• ACM: middle cerebral artery
• AOU: Unique Umbilical artery
• BIP: bi-parietal diameter
• CA: abdominal circumference
• CC: nuchal translucency
• CT: chest circumference
• DAM: mean abdominal diameter
• DAT: transverse abdominal diameter
• DOF: occipito-frontal diameter
• DDR: date of last menstruation
• DPG: date of presumed start of pregnancy
• DS: standard deviation
• LA: amniotic fluid
• LCC: cranio-caudal length
• CSF: cerebrospinal fluid
• LCS: cerebrospinal fluid
• LF: femoral length
• OE: external orifice (cervix)
• OI: internal orifice (cervix)
• OPN: clean bone of the nose
• PA: abdominal perimeter
• PC: cranial perimeter
• EFP: estimated fetal weight
• IUGR: Intra-Uterine Growth Delay
• SA: Week of Amenorrhea
• TM: Time / Movement
• VV: vitelline vesicle
• VO: umbilical vein

Ultrasound helps monitor the baby’s vitality, growth and anatomy. It can thus reveal a lot of information, of which the following are the main ones:

• gestational age
• miscarriage (cessation of heart activity)
• a clear egg
• an ectopic pregnancy
• multiple pregnancy and the type of chorionicity and amniocity
• the baby’s gender
• a suspected genetic abnormality, via nuchal translucency measurement
• IUGR (intrauterine growth retardation)
• -a malformation in the organs
• a malformation of the limbs
• a deformity in the face
• a macrosomia
• death in utero
• a hydramnios
• an oligo-amnion

Like any medical imaging technique, however, ultrasound is not 100% reliable. It can detect a large number of morphological malformations, but not all unfortunately.

Ultrasound can also reveal certain complications on the side of the mother-to-be and the embryo’s annexes:

• a retroplacental hematoma (HRP)
• a placenta praevia
• a low inserted placenta (PIB)
• placentomegaly
• an abnormally small placenta
• a chorioangiome
• a subchorial cyst
• a placental abruption
• a single umbilical artery (AOU)
• a cervical open bite