Ultrasound in pregnancy or obstetric ultrasound is a safe, non-invasive and accurate means of evaluating the fetus. It has become an indispensable obstetric tool and plays an important role in the care of every pregnant woman.
The fetal evaluation may be performed as either part of a basic exam (level I) or a detailed scan (level II).
A basic or level I scan does not need to make a specific diagnosis but is usually able to tell when something appears wrong, resulting in a referral for a detailed or level II scan.
Level I ultrasound is usually carried out in the first trimester of pregnancy. Level II is performed in the second trimester of pregnancy (around 18-20 weeks of gestation).
Historically, the concept of ‘levels’ in obstetric sonograms originated by way of maternal serum alpha-fetoprotein (MSAFP) screening programs. These programs described two levels of sonography with each having specific goals. Level I was performed mainly to detect obstetric problems that resulted in high MSAFP levels (twins, erroneously dated pregnancies, fetal demise) while the level II scan was done to detect fetal anomalies (open neural tube defects, abdominal wall defects, etc) that these programs were designed to detect.
The information that a particular scan provides is determined by the gestational age in which it is carried out. A level II scan, since performed at a later stage, when most of the fetal organs have formed is able to provide greater and detailed information about the growing fetus.
How to Prepare for an Ultrasound Examination?
When carried out in early pregnancy, the urinary bladder should be full. Sound waves travel better through the liquid, so a full bladder enhances the quality of the ultrasound.
A full urinary bladder is however not required for level II scan because, at 18-20 weeks of pregnancy, the baby is big enough and high enough in the abdomen to be seen clearly by a scan conducted through the abdomen.
The abdominal area needs to be exposed for performing the scan. So it is better to wear loose and comfortable two-piece clothing. This will allow the abdomen to be easily accessed by the doctor without having to remove any clothing.
Transvaginal ultrasound is an internal ultrasound in which the ultrasound probe is inserted into the vagina.
It provides very useful information including:
- Documentation of fetal life
- To confirm that pregnancy is in the uterus and not fallopian tubes ie, rule out ectopic pregnancy
- Determination of the number of fetuses
- Determination of gestational age
Ultrasound carried out at around 6-8 weeks to confirm and accurately date pregnancy is known as pregnancy dating ultrasounds or booking scans
- Acquisition of fetal biometric data
- Estimation of amniotic fluid volume
- Recognition of uterine fibroids, ovarian masses, etc
Ultrasound ‘soft’ markers for chromosomal abnormalities such as the absence of fetal nasal bone, an increased fetal nuchal translucency (nuchal scan) indicate an increased risk for Down syndrome or other chromosomal abnormalities. These in combination with the mother’s age and the levels of biochemical markers in mother’s blood (alpha-fetoprotein, beta-human chorionic gonadotropin, pregnancy-associated plasma protein-A) are able to detect the majority of cases of Down syndrome.
Read more about Genetic inheritance: Modes and Significance
Read more about Alpha fetoprotein – Structure Function and Uses
Nuchal scan for assessing nuchal translucency (NT)
It is carried out at 11–13 weeks + 6 days of pregnancy. It assesses the quantity of fluid collecting within the nape (back) of the fetal neck.
In the case of chromosomal abnormalities (including Down syndrome) and major congenital heart problems, the skin at the nape of the neck may become edematous or filled with fluid resulting in increased nuchal translucency.
This parameter is useful only when assessed between the above mentioned time period because during this time, the lymphatic system of the fetus is developing and the peripheral resistance of the placenta is high. After this period, the lymphatic system usually gets developed sufficiently and so any excess fluid is drained away. Also, changes in placental circulation result in a drop in peripheral resistance. So after 14 weeks, if the fetus has any abnormality (which causes excess fluid production), the fluid will not accumulate and the abnormality will not be detected by nuchal scanning.
While the main purpose of level I scan is not to detect fetal anomalies, most of the time during the course of data acquisition, fetal anomalies, if present will be recognized.
Level II ultrasound
(Also known as targeted ultrasound, advanced ultrasound, level II sonogram, fetal anomaly scan)
It is performed in the second trimester of pregnancy (around 18-20 weeks of gestation).
All pregnant women are asked to undergo level II ultrasound. Even if the expecting mother has had a first-trimester (level I) ultrasound to confirm or date the pregnancy, or as part of a first-trimester screening test and no abnormality has been detected, the more detailed level II sonogram is still significant.
The main purpose of this test is to assess the fetal anatomy and detect the presence of any fetal anomalies. It gives detailed information about the growing fetus.
Besides confirming normal fetal growth, fetal movements, twin or multiple pregnancies, the following parameters are assessed.
Evaluation of major organs of the fetus to detect fetal anomalies
Abnormalities like hydrocephalus, anencephaly, hydrocephalus, anencephaly, myelomeningocoele, achondroplasia and other dwarfisms, spina bifida, exomphalos, gastroschisis, duodenal atresia, fetal hydrops, cleft lips/ palate and congenital cardiac abnormalities can be accurately diagnosed on ultrasound. Also, the majority of chromosomal abnormalities like Down’ syndrome, Edwards’ syndrome, Patau’s syndrome, etc can be diagnosed.
- fetal skull (for integrity and shape)
- fetal brain
o ventricles and choroid plexus
o posterior fossa, including measurements of transcerebellar diameter and cisterna magna
o nuchal fold thickness
- profile and nasal bone
- orbits and lenses
- upper lip and palate
Fetal heart and chest
- fetal heart rate and rhythm
- cardiac situs
- four-chamber view
- outflow tract views
- aortic and ductal arches
- diaphragm and lungs
- stomach (including situs)
- kidneys and renal arteries
- abdominal wall
- umbilical cord insertion
- bladder and umbilical arteries
- presacral space
Fetal musculoskeletal system
- both upper and lower limbs
Assessing fetal biometric data
- fetal head – biparietal diameter (BPD) and head circumference (HC).
- fetal abdomen – abdominal circumference (AC).
- fetal leg – femur length ( FL).
- fetal arm – humerus length (HL)
- the distance between fetal eyes – binocular distance (BD).
These measurements are combined to calculate the following parameters
Average gestational age of the fetus
Estimated fetal weight (EFW)
It is of less significance at this stage of pregnancy and assumes importance in the later stages of pregnancy or if premature delivery is imminent.
Estimated date of delivery (EDD)
The EDD calculated on the basis of these parameters has an error margin of +/- 7 days.
Position of the placenta
The position of the placenta is seen in relation to the cervix. If the lowest edge of the placenta is very close to the cervix or lies over the cervix, it is termed low-lying or placenta praevia. Most of the low lying placentas move up and away from the cervix towards the end of pregnancy and do not require any treatment. The position of the placenta is again reviewed in the third trimester as low lying placenta can be a source of heavy bleeding.
This includes the number of cord vessels and evaluation of knots
- Fetal lie
- Estimation of amniotic fluid volume
- Checking of cervix and measurement of the birth canal
- Recognition of uterine fibroids, ovarian masses, adnexa, etc
- Fetal gender
- In some countries, revealing the sex of the child to the parents or family is illegal and punishable by law.
3D/4D images of the baby
Although the main objective of this scan is to check for fetal well-being, 3D/4D pictures of the fetal face obtained during the scan provide special joy and feeling of bonding to the parents.
Follow up After Ultrasound
If the ultrasound findings are normal
A normal result is reassuring and rules out the presence of most of the structural and congenital fetal abnormalities. However, it is possible that in a very small percentage of cases, ultrasound is not able to pick up the abnormality even after a thorough examination.
A normal ultrasound does not guarantee that the baby will be normal or that no complications will develop during the pregnancy. Also, conditions such as cerebral palsy, learning difficulties or autism cannot be detected by ultrasound, as they are not associated with structural changes in the baby.
Most babies (more than 95%) will have completely normal ultrasounds. In some cases repeat scanning may be required after a week or two. This is mostly done in cases where the doctor is not able to see certain details because the fetus was lying in such a position that made few organs inaccessible for viewing.
If Abnormalities are detected on ultrasound
Changes detected in the fetus may fit into the following categories:
Insignificant or Small changes or variations
These usually have no significance and no further follow-up is needed. Examples include conditions like echogenic intracardiac focus (a small white spot in fetal heart, which does not affect the function of the heart) or choroid plexus cysts (small cysts within the choroid plexus of the fetal brain, which does not affect the function of the brain)
In some cases, the umbilical cord has only one artery and one vein (as opposed to 2 arteries and one vein present normally). Such cases require additional ultrasounds later in pregnancy as the single umbilical artery is sometimes associated with small babies.
Some of these differences are also known as soft markers. This means that these features may be seen in normal babies, but they occur more frequently in babies with certain chromosome abnormalities. Soft markers for Down syndrome include echogenic intracardiac focus, shortened femur, and dilation of the collecting system of the kidney. The presence of multiple such soft markers increases the risk for a chromosome abnormality and further testing may be suggested.
Abnormality with minor significance
In some cases, certain abnormalities are detected which although not life-threatening, may require surgery in the neonatal period (such as cleft lip) or other intervention like physiotherapy/ surgery (such as clubfoot or talipes).
Significant, severe or life-threatening abnormality
In a few cases, ultrasound findings may indicate a major abnormality, such as chromosomal disorders, heart defects or spina bifida. These conditions might be associated with pregnancy loss, stillbirth, or significant developmental problems if the baby survives.
Such cases usually require further testing for confirmation and action. These include
A small amount of fluid from around the baby (amniotic fluid) is taken and analyzed for chromosomal abnormalities.
Read more about Amniocentesis- Indications, Procedure, and Complications
Chorionic villus sampling
In this, a small piece of the placenta is taken and examined for chromosomal abnormalities.
NIPT (Non-invasive prenatal testing)
It is performed on the mother’s blood. Cell-free fetal DNA released by the placenta into the maternal blood is collected and studied for genetic abnormalities in the fetus.
Referral to specialists and further testing
A fetal echo scan for a detailed examination of the fetal heart should be done for heart problems.
Corrective treatment before and after birth
Some conditions may require corrective surgery or treatment after birth, or in select cases, surgery may be carried out when the fetus is still in the uterus.
Termination of pregnancy
In a small percentage of cases, it may be realized that the fetus will not be able to survive the postneonatal period or if it survives, will have severe mental and physical abnormalities. In such cases, the option of medical termination of pregnancy should be offered to the parents and family. The mother, in addition, must be offered psychological and emotional support.