How many ultrasounds should be done during pregnancy? What should be checked each time?

Author: Wu Qingqing, Chief Physician, Beijing Obstetrics and Gynecology Hospital, Capital Medical University

Reviewer: Bai Wenpei, Chief Physician, Beijing Century Altar Hospital, Capital Medical University

Every pregnant mother will have an ultrasound examination during the prenatal check-up because the baby can be seen under ultrasound and even what the baby looks like. Ultrasound examination has no radiation and has no harm to the fetus.

Routine ultrasound examinations during pregnancy generally require 5-6 times, which means that these 5-6 times are necessary. The specific number of times each pregnant woman needs to do it depends on the specific situation of the pregnant woman and the fetus.

Next, let’s talk about when ultrasound examinations during pregnancy are performed and what they specifically check.

1. First Ultrasound Examination

It is usually done around 42 days of pregnancy to see whether it is an intrauterine pregnancy or an ectopic pregnancy, to check the number of embryos, and whether there is a fetal heartbeat or fetal bud.

Figure 1 Original copyright image, no permission to reprint

2. Second Ultrasound Examination

It is usually performed at 11-13+6 weeks of pregnancy and is a relatively important ultrasound examination. This ultrasound examination is mainly used to verify the gestational age of the fetus and measure the NT value.

NT (nuchal translucency) refers to the thickness of the fetal nuchal translucency, that is, the thickness of the fetal nuchal translucency is measured when the head-to-rump length is 45-84mm.

Figure 2 Original copyright image, no permission to reprint

NT value <2.5mm is within the normal range. NT value ≥2.5mm is abnormal, indicating that the fetus may have chromosomal abnormalities, such as trisomy 21, trisomy 18, and trisomy 13, but it is not 100% certain to occur, but the risk of chromosomal abnormalities will increase. In addition, NT thickening is also related to abnormalities in the fetal heart and skeletal system, and the risk of congenital heart disease and skeletal abnormalities increases.

Whether an abnormal NT value requires prenatal diagnosis depends on the pregnant woman's age, clinical history, and other high-risk factors to determine whether chromosome testing, such as amniocentesis or chorionic villus sampling, should be performed.

3. The third ultrasound examination

It is usually done between 20-24+6 weeks of pregnancy, preferably between 22-24 weeks. Ultrasound is used to perform systematic structural screening of the fetus, mainly to screen for gross malformations of the fetal head, chest, abdomen, limbs, spine and other organ structures to see if the organs and structures are fully grown and if there are any problems. This screening is the most important examination, commonly known as major malformation screening.

Figure 3 Original copyright image, no permission to reprint

The head mainly checks whether the fetal skull and intracranial structures, such as the cavum septum pellucidum, thalamus, cerebellum, lateral ventricles, and cerebellomedullary cisterna, are normal.

The chest is mainly examined to check the lungs and heart of the fetus, such as whether there is anything growing in the lungs, whether the four-chamber heart and outflow tract are normal, and whether there are any abnormalities in the heart structure.

The abdomen is mainly examined to see whether there are any abnormalities in the fetus' liver, kidneys, stomach, intestines, and bladder.

The limbs mainly focus on whether the fetus's upper and lower limbs are deformed or missing. At present, it is not possible to count the fetus's fingers and toes because the fetus often makes a fist or is blocked, making it difficult to count the fingers and toes.

The spine mainly checks whether the fetal spine is well aligned and whether there is spina bifida.

Ultrasound screening for fetal malformations can screen out some serious, lethal malformations that currently have no good treatment methods after birth, such as anencephaly and open spina bifida. Pregnant women can choose to terminate their pregnancy with informed consent before 28 weeks. Screening out minor malformations that can be treated after birth can provide regular monitoring and guidance during pregnancy.

At present, ultrasound examination cannot screen out all fetal malformations. Even the best ultrasound doctors in the world, using the best ultrasound equipment in the world, still cannot screen out all malformations. Ultrasound screening mainly screens large fetal structures. Small structures, such as small ventricular septal defects, aortic valve stenosis, accessory ears, and polydactyly are difficult to screen out.

In addition, ultrasound cannot screen for chromosomal abnormalities or genetic diseases, but ultrasound can detect some soft genetic markers, such as NT thickening, which indirectly indicates a high risk of chromosomal abnormalities. For example, choroid plexus cysts are also markers of chromosomal abnormalities. The same is true for genetic diseases. When looking at the facial contours of the fetus, it is found that the nose and facial contours are particularly flat. This situation may be related to genetic diseases. Ultrasound cannot determine what kind of genetic disease it is. Blood tests and amniocentesis can be used to detect whether there are abnormalities in genes and chromosomes to make up for the shortcomings of ultrasound examinations.

Finally, is four-dimensional ultrasound used for major fetal malformation screening?

All over the world, including the International Society of Ultrasound in Obstetrics and Gynecology, guidelines and industry requirements, use two-dimensional ultrasound for real-time dynamic malformation screening. Four-dimensional ultrasound is not routinely used for fetal malformation screening. It is only when a problem is found in a fetal organ during the screening process, such as a problem with the fetal heart, that three-dimensional or four-dimensional ultrasound technology may be used to image the fetal heart for more detailed observation. Three-dimensional and four-dimensional ultrasound are supplementary examinations to two-dimensional ultrasound.

4. Fourth Ultrasound Examination

It is usually done between 28 and 32 weeks of pregnancy, and can be done up to 34 weeks. This ultrasound examination is mainly for a missed screening and secondary malformation screening to see if the size of the fetus is consistent with the actual gestational age, commonly known as small malformation screening. In addition, the placenta, amniotic fluid, umbilical blood flow, etc. will be monitored to determine whether the fetus is in distress or hypoxia.

Why do we need to conduct a secondary fetal malformation screening? Because the fetus will also undergo a series of changes during its growth. For example, at 22 weeks, the size of the fetal kidney is consistent with the gestational age, which is normal. However, at 30 weeks, the growth of the kidney may not keep up with the corresponding gestational age, which is equivalent to the size of 25 or 26 weeks. Kidney malplasia may occur, so this can be discovered in time.

5. 5th-6th Ultrasound Examination

It is usually done between 34 weeks of pregnancy and before birth. It uses ultrasound to monitor umbilical blood flow, amniotic fluid, placental maturity, fetal position, etc. It mainly observes the condition of the fetus in the uterus, whether there is hypoxia, etc., and can also estimate the approximate weight of the fetus.

The above are the routine ultrasound examinations that need to be performed throughout the pregnancy.

For pregnant mothers, each ultrasound examination is like passing a level. After passing one level after another, our baby will be born.