Pregnancy ultrasounds can be very useful and provide a lot of information, but it is important to remember that they also have their limitations.
The accuracy of a pregnancy ultrasound will very much depend on the qualifications, skill and experience of the ultrasonographer performing the examination, as well as their skill at interpreting the images. However.....
.....even the most experienced and skilled pregnancy ultrasonographers with the absolute best equipment can still miss things and misinterpret images, just less often!
Accurate ultrasound findings also depend on the stage of the pregnancy and what the ultrasound examination is trying to achieve. For example, it is very easy to determine the position of an unborn baby, but it requires a very experienced ultrasonographer to detect pregnancy problems or physical abnormalities in the baby. Even then, detecting some abnormalities is not always possible and in up to 10% of cases, a baby will be mistakenly thought to have an abnormality, but in fact be well and healthy.
NOTE: Some maternity caregivers purchase ultrasound machines to perform 'informal' ultrasounds at every pregnancy visit in their private rooms. (This is not an essential part of routine pregnancy visits and you can decline having this done if you prefer.) Generally, most maternity caregivers have very limited training in the use and interpretation of ultrasounds and can only provide a small amount of information. You would need a formal ultrasound performed by a qualified ultrasonographer (technologist) and reported on by a qualified sonologist (specialist doctor) to provide more detailed information and/or definitely confirm any possible findings by your maternity caregiver.
What can ultrasounds show?
An ultrasound can reveal the size of the baby after 6 weeks (and therefore estimate their gestational age). Later ultrasounds (after 18 weeks) can show the position that the baby is lying in and detect many obvious physical abnormalities (such as a cleft lip or a heart defect). Ultrasounds cannot reveal a baby's genetic 'make-up' and this type of information can only be obtained through genetic tests . However, up to 40 to 50% of babies with genetic disorders will be detected through 'soft signs' picked up by the ultrasonographer after 18 weeks (such as a thickened fold of skin at the back of the baby's neck, which may or may not indicate a genetic disorder). Ultrasounds are also able to show the location of the placenta and possibly shed light on other issues such as the cause of bleeding during pregnancy or detecting multiple pregnancies (twins or more).
Ultrasound images are generally limited by the scope of the probe and where the sound waves can reach. Therefore during the first half of pregnancy, the whole baby can be seen on the screen, often moving their arms and legs, but the details of their face are usually less distinct. During the last half of pregnancy, the whole baby cannot be viewed all at once, but you can usually see clearer images of their facial features, hands or feet.
Reasons for ultrasounds
Your caregiver may order an ultrasound (or more than one) at various times during the pregnancy, for different reasons. If the pregnancy is progressing normally, routine ultrasounds are not compulsory (and some women do decline them). However, the most common routine ultrasound is usually scheduled for around 18 to 20 weeks of the pregnancy and is aimed at screening for abnormalities in the baby.
Some reasons an ultrasound may be performed can include:
Confirming a pregnancy
A pregnancy is usually confirmed with a urine or blood pregnancy test. However, if your caregiver is concerned about a possible miscarriage (because of vaginal bleeding) or perhaps an ectopic pregnancy, they may also order an ultrasound to visually see the pregnancy and estimate how advanced it is (as well as checking that all is going normally). An ultrasound may also be ordered to confirm a viable, ongoing pregnancy for babies conceived using fertility treatments.
Threatened or suspected miscarriage
If you are experiencing vaginal bleeding before 20 weeks of the pregnancy (possibly indicating a miscarriage) your caregiver may order an ultrasound (or a couple of ultrasounds over a period of a week or so), to check for the baby's heart beat (if more than 6 weeks) and to see if the pregnancy is progressing normally. Ultrasound tests may also be combined with blood tests to gain a better idea of what may be going on.
As a general guide:
It is possible to detect a pregnancy of 5 ½ weeks gestation on ultrasound. A tiny sac can be seen, but the baby and their heart beat may not be detected yet. Be aware that while it is possible to see a pregnancy this small, in many cases the pregnancy may not be detected, because the sac is so tiny. You may need to have another ultrasound a week or so later to detect the pregnancy, if you still feel you are pregnant and/or your blood HCG levels are rising.
A baby can be seen at 6 to 7 weeks and their heart beat may be detected at this early stage (90 to 110 beats per minute under 6 to 7 weeks, then 110 to 200 beats per minute as the baby matures). Again, the ultrasound may need to be repeated a week or so later to see if the baby is growing.
Between 7 and 8 weeks, the baby and their heart beat can both be detected relatively easily. This is presuming that the pregnancy is actually at this stage of development and not less advanced than you thought it was.
After 8 weeks the baby and heart beat can be clearly seen. From this point onwards, ultrasounds tend to be more likely to be used to monitor the wellbeing of the baby. The ultrasound may be repeated a few days or week or so later to check the baby's growth and progress.
If the pregnancy is far enough advanced for the baby's heart beat to be detected and the woman is experiencing bleeding, an ultrasound showing the baby is alive will in most cases (85%) be indicative that the pregnancy will still progress normally. However, for up to 15% of women, even if the ultrasound showed that their baby was alive, they will still go on to miscarry at some stage.
An ectopic pregnancy (or 'out of place' pregnancy) occurs when the woman's egg is fertilised but implants before it reaches her uterus. Most ectopic pregnancies involve the baby implanting in the fallopian tubes, (about 98%) but in a few cases, the baby will implant outside the woman's uterus on her ovary or in her lower abdomen. It is estimated that about 1% of pregnancies are ectopic.
If your caregiver suspects an ectopic pregnancy, they will usually order an ultrasound to see where the baby is (as well as a blood pregnancy test for pregnancy hormones). The ultrasound will often involve having a vaginal ultrasound, to enable the ultrasonographer to look more closely at the uterus and fallopian tubes to help find the baby.
'Dating' the pregnancy
'Dating' the pregnancy (or estimating the gestation of the baby and therefore their due date) is usually based on the first day of the woman's last menstrual period. However, a 'dating' scan may be ordered if the woman:
Has had irregular periods or her menstrual cycle has been disrupted because she has recently stopped hormonal contraception, or is breastfeeding or has recently experienced a miscarriage.
Does not know when the starting date of her last period was and/or her baby's probable conception date.
Experiences a light 'implantation bleed', which may have been confused with her last period date.
Ultrasounds can be of great help in determining how far pregnant a woman is, particularly if she has no idea herself. However, ultrasounds are not always accurate at calculating the gestation of a baby, because they rely on measuring the baby's physical size. (Their 'crown to rump' length during early pregnancy and their femur (or thigh) length and head size, as they grow older.) 'Due date' estimations are based on the 'middle average' size of babies, but individual babies differ in size depending on their genetic make-up (as all human beings do). Therefore they are not always accurate. Generally speaking:
...the earlier the ultrasound is done, the more accurate it can be at estimating the baby's due date.
As a guide:
Ultrasounds performed during the first 12 weeks of pregnancy (preferably between 8 and 12 weeks) are generally within 3 - 5 days of accuracy. This is because the baby is growing extremely rapidly, and there is a great deal of difference between the sizes of a 7, 8 or 9 week old unborn baby. Some caregivers will recommend an early ultrasound to help calculate the woman's due date (however, if you are sure of your dates, then a dating ultrasound is not necessary). If you have an ultrasound done before 12 weeks of pregnancy and your baby's age is estimated to be more than +/- 7 days from the original due date, your caregiver will probably readjust when your baby is due.
As the baby grows, the differences in their growth stages from week to week become less obvious, reflecting the individuality of each woman's baby. Ultrasounds from 12 to 22 weeks are regarded as being within 10 days of accuracy (or up to 10 days earlier or 10 days later than the woman's calculated due date). If you have an ultrasound during this time and your baby's age is estimated to be more than +/- 10 days from the original due date, your caregiver will probably readjust your due date.
Ultrasounds performed after 22 weeks can be up to 2 to 3 weeks out, and as a general rule should not be used to estimate the due date of your baby (unless this is all you have to base your due date on). If you have more than one ultrasound during your pregnancy, giving you 'multiple dates', then the earliest ultrasound estimate should be used.
Other aspects that can affect the accurate calculation of the baby's age include:
The operator's qualifications, skill and experience at performing and interpreting pregnancy ultrasounds.
Measuring the 'gestational sac' volume during very early pregnancy (less than 6 to 7 weeks), which is less accurate than measuring the baby's 'crown to rump' length.
The time taken to perform the scan. Faster examinations tend to be less accurate than if the operator takes time to obtain precise measurements.
The type of equipment used (digital machinery is better then analog) as well as the software used to estimate dating statistics.
Genetic tests are used to screen for and detect specific inherited disorders in unborn babies. Many of the genetic tests available involve the use of ultrasound. These can include:
A nuchal translucency or 'NT'
A nuchal translucency uses ultrasound to visualise and measure a fluid filled sac at the back of the unborn baby's neck during early pregnancy. NT is usually performed between 11 weeks + 3 days and 13 weeks + 6 days to estimate if the baby is at increased risk of having a chromosomal abnormality such as Down syndrome (also called 'trisomy 21').
A 'chorionic villus sampling' (or CVS)
A CVS takes a small sample of cells from the baby's placenta and examines them to 'map' the baby's genes or chromosomes with the aim of identifying the presence of a genetic disorder (such as Down syndrome). Ultrasounds guide the caregiver so that they place the needle in the correct location and avoid the baby. A CVS is usually performed between 10 and 12 weeks of pregnancy.
An amniocentesis (or 'amnio')
An amniocentesis takes a sample of amniotic fluid that the baby floats in from inside the woman's uterus to obtain cells naturally shed from the baby floating in the fluid. Like CVS, an amniocentesis aims to identify the presence of a genetic disorder (such as Down syndrome). Ultrasound is used to guide the caregiver so they place the needle in the correct location and avoid the baby. An amniocentesis is usually performed at about 15 weeks of pregnancy, but can be performed up to 18 weeks or even later than this.
A cordocentesis takes a blood sample from one of the blood vessels in the baby's umbilical cord before birth, to directly test the baby's blood for genetic disorders. It involves the doctor using ultrasound images to guide the insertion of a long, thin needle through the woman's belly and uterus and into the umbilical cord. A small blood sample is then taken and the needle is removed. Cordocentesis can only be done later in the pregnancy, from about 18 - 24 weeks, when the umbilical cord has adequately developed.
Detecting twins or more
If your caregiver (or you) suspects you may be carrying more than one baby, they will probably order an ultrasound to see if this is the case. Ultrasounds may also be performed more often during a multiple pregnancy to check how the babies are growing and what position they are lying in to plan for the birth.
Screening for abnormalities
The most common routine ultrasound is usually scheduled for around 18 to 20 weeks of the pregnancy and is primarily aimed at screening for obvious physical abnormalities in the baby. The medical term given to this ultrasound is a 'fetal anomaly scan' or a 'fetal morphology scan' and it is generally regarded as the most involved ultrasound that can be done during pregnancy.
Ultrasounds are not able to reveal a baby's genetic 'make-up' (this can only be obtained through genetic tests). However, up to 40 to 50% of babies with genetic disorders will be picked up by an ultrasound examination. Even so, it is important to remember...
........ultrasounds cannot exclude every possible problem in an unborn baby and not all birth defects can be detected by an ultrasound.
The baby must be at least 18 weeks gestation for the ultrasonographer to adequately examine all the baby's body systems to screen for abnormalities. However, some abnormalities become more obvious as the baby becomes larger and more developed. For example, the baby's heart is easier to examine thoroughly at around 22 weeks. If the ultrasonographer suspects a possible problem, then a more detailed ultrasound of the baby is usually scheduled at a later time, to specifically examine the baby for the abnormality in question.
The routine '18 to 20' week timing came about because caregivers wanted to strike a balance between picking up as many possible abnormalities as early as possible during pregnancy and hopefully allowing parents more choice, particularly regarding whether they wish to proceed with the pregnancy if an abnormality is detected.
NOTE: Be aware that ultrasounds can have 'false positive' results. This means the ultrasound may indicate the baby has a problem, but is in fact normal and healthy. Depending on the condition 'identified', the experience of the operators and the equipment used, up to 1:10 pregnant women (10%) can be told that their baby has a birth defect, when in fact their baby does not.
As a summary, an anomaly scan involves the ultrasonographer looking at:
The woman's uterus, cervix and placenta, noting the proximity of the placenta to the cervix to see whether it is low lying.
The amount of amniotic fluid surrounding the baby and estimating whether the volume is within the normal expected range. Too much fluid is known as 'polyhydramnios', too little is known as 'oligohydramnios' .
Both ends of the baby's umbilical cord and counting the blood vessels in the cord, of which there should be 3 vessels (2 arteries and 1 vein). Sometimes there will only be 2 vessels (one artery and one vein).
The position the baby is lying inside the uterus (known as the 'presentation'), for example, breech (bottom first), transverse or head down (called 'cephalic').
The baby's skeleton, arms, legs, hands and feet. Checking to see that their feet are not clubbed. The ultrasonographer will also note if the baby is moving during the examination.
The baby's genitals (if able to be seen). They may be able to let you know the sex of your baby, if this can be identified (and you want to know).
The baby's face, eyes, nose, tongue and mouth (to look for cleft lip).
The baby's stomach, bladder, diaphragm and kidney's, particularly the section of the kidney that collects urine for transport to the bladder (called the 'renal pelvis'). The renal pelvis is measured (if seen adequately) and if it is excessively dilated this may possibly indicate future kidney problems.
The baby's femur (or thigh bone) length is measured as well as their abdominal circumference, biparietal diameter (width of their head) and head circumference (measurement around their head). These may need to be measured 2 or 3 times, depending on how clearly the areas are able to be seen. All the measurements are put into a software program to estimate the gestation age of the baby.
The baby's skull and spine are carefully examined for neural tube defects and their brain is examined and measured. The ultrasound may reveal the presence of possible choroid plexus cysts.
The baby's abdominal wall and intestines are examined.
The baby's heart is examined, particularly the 4 chambers of the heart, the partitions (or septums between the chambers) to try and detect a hole in the heart. The heart valves are checked and the direction of blood flow through the main heart vessels (the aorta and pulmonary artery) as well as counting the actual heart rate.
NOTE: Be aware that the position of your baby can hinder the ability of the ultrasonographer to capture all the desired images of your baby. They may recommend repeating the ultrasound in 30 minutes to an hour (or on another day), to see if the baby has turned to a more favourable position to view what is required for the examination.
Bleeding during late pregnancy
If you are experiencing vaginal bleeding after 20 weeks of pregnancy, your caregiver will probably order an ultrasound to try and reveal the cause of the bleeding. Concerns are that the placenta may be lying low (called 'placenta previa') or that there may be bleeding behind the placenta (called a 'placental abruption'). Sometimes the ultrasound is unable to clearly identify any cause for the bleeding.
Occasionally, a caregiver will suspect that a baby is not growing as well as expected, (usually between 24 and 36 weeks of pregnancy). This is referred to as 'small for gestational age' (or SGA). If this is the case they may order several ultrasound examinations at regular intervals over a period of weeks or months (referred to as 'serial ultrasounds'), to assess the ongoing growth and wellbeing of the baby as the pregnancy progresses.
Aspects that the ultrasonographer may look for include:
The progressive growth of the baby. The baby's head, abdomen and femur (or thigh bone) length are measured, to estimate the baby's size. Measuring the baby's growth is only of value if a few ultrasounds are done at regular intervals, to reflect a pattern of growth. A 'one-off' ultrasound is of limited value to decide if a baby is not growing well.
Any evidence of a 'head sparing effect'. If a baby is not receiving sufficient blood flow through their placenta, they automatically channel most of their required nutrients to their brain, at some expense of their body. This means that the baby's head grows normally, but their body grows more slowly (called a 'head sparing effect'). The ultrasonographer measures the baby's head circumference (HC) as well as their abdominal circumference (AC) and plots them on a graph. If the graph reflects the baby has a larger head for their age, compared to their body size, then this is a fairly accurate measure of a 'small for gestational age baby'.
The baby's weight. Ultrasound can sometimes be used to estimate the weight of the baby, but this is not a very accurate tool. Estimates can be wrong by up to 10 to 15% or +/- 500 to 700 grams (1lb 2oz to 1lb 9oz) and it is generally recommended that estimates of this type should not be relied on.
The amount of fluid around the baby can be measured and estimated by using ultrasound. Amniotic fluid is produced mainly by the membrane of the placenta and is swallowed and passed through their kidneys as a type of 'urine' by the baby. If the placenta is not functioning at it's best the amount of amniotic fluid can be reduced, as well as the baby's kidneys possibly receiving less blood flow, and producing less urine.
Measuring the flow of blood through the umbilical cord. This is referred to as 'Doppler studies' and aims to assess if the cord has adequate blood flowing through it.
Looking for possible placental problems. These could include the position of the placenta such as placenta previa, a blood clot behind the placenta from a bleed, or detecting parts of the placenta that are not receiving any blood flow.
To detect possible abnormalities of the baby that may be causing their growth to be impaired. (Ultrasounds cannot detect all abnormalities or genetic disorders, but may be able to detect some).
NOTE: It is now well recognised that performing routine ultrasounds on every woman to try and detect babies that are not growing well are not justified, because the hands of an experienced midwife or doctor feeling a pregnant woman's belly and measuring the height of the uterus (or 'fundal height'), can just as accurately detect small babies. Using ultrasound to try and detect abnormally small babies has been shown to have a high 'false-positive' rate (over 50%). This means that many perfectly healthy babies will be suspected of being unwell, leading to increased anxiety for the parents, as well as an increase in the use of unnecessary interventions.
Sometimes, a caregiver will suspect that a baby is abnormally large (usually after 36 weeks of pregnancy). This is referred to as 'large for gestational age' (or LGA). The caregiver may order an ultrasound examination to try and estimate the size of the baby. However, this method has been shown to be up to 10 to 15% out, or +/- 500 to 700 grams (1lb 2oz to 1lb 9oz), even when performed by a highly experienced ultrasonographer. Therefore it is generally recommended that estimates of this type should not be relied on.
Checking the placental position
During a routine 18 to 20 week scan, the ultrasonographer checks the position of the placenta and comments on where it is situated. In most normal pregnancies, the placenta implants high in the top of the uterus (known as the 'upper segment' or 'fundus'), either a little towards the back (posterior), or a little towards the front (anterior), or towards the right or left (lateral). Occasionally, the ultrasound will show that the placenta is 'lying low', meaning that the placenta is situated closer down, near the woman's cervix.
A low lying placenta at this early stage of the pregnancy is not abnormal and is evident in about 5% of all ultrasounds. This is because the lower segment of the uterus does not form until about the 24th week of pregnancy and is fully formed by 36 weeks. During this time, the uterus slowly stretches upwards, forming the thicker upper segment of the uterus and leaving a thinner layer of muscle below it, known as the 'lower segment' of the uterus, before it reaches the woman's cervix. The lower segment is the part of the uterus that absorbs the cervix as it dilates during labour.
In most cases (about 90%) the previously low lying placenta moves up and out of the way into the upper segment as the uterus grows, meaning that by about the 34th to 36th week, it is no longer low lying. On rare occasions, (in 0.5% of pregnancies), the placenta remains in the lower segment of the uterus, or has itself grown to the extent that it covers the cervix. This is known as 'placenta previa' , meaning 'placenta first'.
NOTE: Ultrasounds are not 100% accurate and it can sometimes be difficult to tell if a placenta is actually low lying or not. That is, an ultrasound may question the presence of a low-lying placenta, even though it is situated normally, or the ultrasound cannot tell that the placenta is low lying. Ultrasounds done through your belly are generally regarded as less accurate at diagnosing placenta previa, because the cervix is situated very low down in the woman's pelvis. Ultrasounds done with a probe into the woman's vagina are suggested as being more accurate for diagnosing placenta previa.
Detecting the way the baby is lying
In most cases, after about 24 weeks, an experienced maternity caregiver can detect the way an unborn baby is laying inside the uterus. They do this by feeling for the baby's head through the woman's pregnant belly (called 'palpating'). However, even the most experienced caregivers can sometimes find it difficult to tell the position of an unborn baby. During the last few weeks of pregnancy (before labour starts) it is important to know which way the baby is lying. Most babies will be in a head down position (called 'cephalic'), but if your caregiver suspects that your baby may be in a breech position (or another unusual position such as transverse), they will probably order an ultrasound to confirm the baby's position.
Ultrasounds are also used by caregivers to guide them when attempting to turn a baby in a breech position to a head down position. This is called an 'external cephalic version' (or ECV), which is usually performed between 35 to 37 weeks of pregnancy.
Baby's wellbeing when overdue
In recent years, ultrasounds have been used to check the well-being of unborn babies once the woman is well past her due date (usually 7 to 10 days overdue).
The tests may include:
Measuring the amniotic fluid volume around the baby, also called an 'amniotic fluid index' (or AFI). However, this is regarded as a controversial practice by some caregivers. Fluid measures can vary, depending on the ultrasonographer and the method used to measure. Also, amniotic fluid volume decreases normally from 37 weeks onwards anyway and factors such as the hydration of the woman can also affect amniotic fluid volume. If the mother is dehydrated then the volume can be lower (so drink plenty of water during the 24 hours before the test!)
A 'biophysical profile' (or BPP). This is a combination of the CTG as well as the ultrasound to measure the amount of amniotic fluid around the baby and assessing the blood flow through the baby's cord and perhaps blood flow between the uterus and placenta. All three aspects are considered when assessing the health of the baby.
Some hospitals look at the baby's breathing pattern and the tone of the baby's posture and movements. However, there is little evidence of the effectiveness of these measures.
Another technique called 'placental grading', which looks at the texture of the placenta using ultrasound. Early research shows this may be a more accurate tool, but it is not conclusive.
NOTE: All the above tests act as a guide only and do not guarantee the health of your baby. There are some women, who have a good AFI or Biophysical Profile (BPP), and yet their baby is not well, and others who have a low AFI or poor BPP and their baby is actually fine. At present we have no accurate, reliable tests available that are able to definitely tell us how the baby is actually coping in the uterus.
Related pregnancy articles:
Last revised: Wednesday, 25 June 2014
This article contains general information only and is not intended to replace advice from a qualified health professional.