What are ultrasounds?
Ultrasounds are sound waves used in medicine to image various internal organs and have been used for the past 30 years. So far, there is no evidence that ultrasounds can cause harm to the fetus.
Why should we have an ultrasound during pregnancy?
How many ultrasounds are needed?
Most babies are healthy; however, about 1–2 babies are born with a serious abnormality or intellectual disability. With ultrasound, we can diagnose approximately 90% of severe abnormalities. Ultrasound also provides information about fetal growth and the position of the placenta.
The ultrasound at 11 to 14 weeks (Nuchal translucency, nasal bone, PAPP-A). One of the most common causes of intellectual disability is Down syndrome. Children with Down syndrome always have some degree of intellectual disability and often have heart problems. Any woman can have a child with Down syndrome, but the likelihood increases as she gets older.
The only way to be sure that the fetus does not have Down syndrome is a diagnostic test (amniocentesis or chorionic villus sampling), in which we examine genetic material from the amniotic fluid (amniocentesis) or the placenta (chorionic villus sampling), which is identical to the genetic material (chromosomes) of the fetus. However, these procedures carry a small additional risk of miscarriage, estimated in recent studies at about 0.1 to 0.3%
For this reason, the decision to undergo such a procedure is best based on a test that estimates the probability that the fetus has Down syndrome. This test is the 11–14 week ultrasound, focusing on the presence of the nasal bone (the fetal nasal bone).
When these findings are combined with the measurement of two maternal blood hormones, we can predict at least 9 out of 10 babies with Down syndrome (sensitivity 95%).
ALPHA-FETOPROTEIN (AFP) TEST, TRIPLE TEST
This test is performed between the 15th and 18th weeks of pregnancy to measure the levels of alpha-fetoprotein (AFP) in your blood. This protein is produced by your baby and passes into your bloodstream during pregnancy. High levels of this protein are associated with spina bifida, and you will therefore be advised to have an ultrasound and an amniocentesis to confirm the diagnosis.
The triple test is a term used for a blood test performed on the mother to detect Down syndrome. It combines the AFP result (which also provides information about the risk of spina bifida) with the measurement of other blood chemicals to estimate the risk of giving birth to a baby with Down syndrome.
The triple test is not useful in the case of twins or multiple pregnancy. Some maternity clinics provide results as “low risk/screen negative” or “high risk/screen positive.” A negative result means that the risk of having a baby with Down syndrome is low. A positive result means that you have a high risk of giving birth to a baby with Down syndrome. For example, any level above 1:250 is considered “high risk.” This is usually the cutoff point. However, a risk of 1:100 still means a 1% chance of the baby having Down syndrome and a 99% chance of being born healthy. You will be advised to undergo an amniocentesis to obtain a definitive diagnosis. You may compare this risk with your age-related risk (approximately 1:700 at age 30) or the risk of miscarriage from amniocentesis (about 1:100).
Amniocentesis / Chorionic Villus Sampling (CVS)
Amniocentesis (amniotic fluid sampling) and chorionic villus sampling (a biopsy taken from the placenta) are invasive methods for checking the fetal chromosomes. Chorionic villus sampling is performed from the 11th week of pregnancy. Under ultrasound guidance, a thin needle is inserted into the placenta and tissue (chorionic villi) is collected.
Similarly, amniocentesis is performed from the 16th week. Under ultrasound guidance, a thin needle is inserted into the amniotic sac to collect amniotic fluid. With both methods, the number and integrity of the chromosomes are examined, allowing major abnormalities — such as Down syndrome — to be ruled out.
However, this testing cannot rule out every abnormality, and a normal result does not guarantee the birth of a healthy child. Both amniocentesis and chorionic villus sampling carry a small additional risk of miscarriage, about 0.1 to 0.3% according to recent studies
The 21–24 week ultrasound and the 4D ultrasound
(Second-trimester anatomy scan)
The second-trimester anatomy scan is performed between the 21st and 24th week of pregnancy and includes a series of measurements to assess fetal growth, evaluate the placenta, estimate the amniotic fluid, and conduct a detailed examination of the fetal organs for congenital abnormalities.
The scan evaluates the skull, brain, face, spine, chest, heart, abdomen, kidneys, bladder, arms, and legs. With this detailed ultrasound, approximately 70% of severe abnormalities, about 45% of milder abnormalities, and overall around 55% of congenital anomalies can be detected. Some abnormalities develop during pregnancy or may appear late in pregnancy or after birth. Unfortunately, no ultrasound — no matter how detailed or comprehensive — can exclude all abnormalities or guarantee the birth of a completely healthy child.
The fetus is also examined for markers (signs) of chromosomal abnormalities. About half of fetuses with Down syndrome have one or more markers, although most fetuses with markers are healthy. The results of the 11–14 week scan are considered together with the first-trimester combined test and the second-trimester anatomy scan when calculating the final probability of Down syndrome. Using color Doppler ultrasound, the blood flow in the uterine arteries is assessed. If the resistance in the uterine arteries is increased, the likelihood of the mother developing preeclampsia or the fetus having low birth weight is higher.
About 1 in 4 women with increased resistance will experience such complications, while the remaining pregnancies will progress normally. If increased resistance is identified in the anatomy scan, regular monitoring of the mother's blood pressure and monthly fetal ultrasound examinations are recommended.
Finally, the length of the cervix can be measured. The cervix is the lower part of the uterus that opens (dilates) during labor. Its measurement is reliably performed via transvaginal ultrasound. If the cervix has significantly shortened (less than 15 mm), the risk of preterm birth increases. In such cases, your doctor will suggest appropriate treatment to reduce the likelihood of this serious complication. Recent studies have shown that such measures can reduce the incidence of very preterm birth by up to 40%.
The 4D ultrasound provides a three-dimensional, live (real-time) image of your baby.
Growth ultrasound and Doppler
The growth ultrasound is performed from 24 weeks until the end of pregnancy, usually every four weeks. It assesses the fetus’s growth, its weight, the quality of the amniotic fluid, and the position of the placenta. The Doppler examination evaluates blood flow in the vessels of the fetus and the placenta (usually the umbilical artery and the middle cerebral artery). It provides information about placental function and the condition of the fetus. These findings are particularly important for growth-restricted babies and help us determine whether a small-for-gestational-age fetus is receiving enough oxygen and nutrients from the placenta. When blood flow in these vessels is not normal, more detailed and frequent monitoring is required.
The first pregnancy ultrasound: 6 to 10 weeks
Before the nuchal translucency scan, most women have a first ultrasound a few weeks after a positive test. It is short, but it answers questions that shape the entire follow-up:
- Where is the pregnancy? It confirms that the gestational sac is inside the uterus and rules out an ectopic pregnancy, which requires immediate treatment.
- Is there a heartbeat? The heartbeat usually becomes visible around the 6th to 7th week. If the ultrasound is done very early, its absence does not necessarily mean a problem, and the scan is repeated in 7 to 10 days.
- How many embryos are there? In twin pregnancies, it is determined early whether the babies share a placenta.
- What is the exact gestational age? Measuring the embryo in the first trimester is the most reliable dating method.
At this stage the ultrasound is often done transvaginally, because the embryo is too small to be seen clearly through the abdomen.
Abdominal or transvaginal ultrasound
Obstetric ultrasounds are done in two ways, and the choice is not random.
- Abdominal: the ultrasound probe moves over the abdomen with a little gel. It is the usual method from the end of the first trimester onward.
- Transvaginal: a thin probe, covered with a protective sheath, is inserted into the vagina. It gives a clearer image in the early weeks, for measuring cervical length and for checking the position of the placenta when it is low-lying.
Transvaginal ultrasound is safe at every stage of pregnancy, does not cause miscarriage and is no more uncomfortable than a routine gynecological exam.
What is measured in the third trimester ultrasound
In the last trimester, the doctor focuses on growth and on preparing for delivery. The key measurements are:
- Biometry: head diameter and circumference, abdominal circumference and femur length. These are used to calculate the estimated weight, with a margin of error of about 10 to 15%.
- Growth percentile: shows how the baby compares with others of the same age. More important than a single measurement is the trend of the curve across successive scans.
- Amniotic fluid: both too little and too much fluid need investigation. Polyhydramnios, for example, is often associated with gestational diabetes.
- Placental position: if it was low-lying at the 20-week ultrasound, it is checked again around the 32nd week. In most women it “moves up” as the uterus grows.
- Fetal presentation: around the 36th week, the doctor checks whether the baby is head down. In a breech presentation, the delivery options are discussed.
How to prepare for each ultrasound
- First trimester: for a transvaginal ultrasound, your bladder should be empty. For an abdominal scan, a glass of water beforehand helps.
- Second and third trimester: you do not need a full bladder or fasting. A light meal often makes the baby more active.
- Clothing: choose a two-piece outfit for easy access to your abdomen.
- Creams: avoid lotion on your abdomen on the day of the scan, as it can affect image quality.
- Documents: bring your previous reports so that measurements can be compared and the trend can be seen.
For an overall picture of what changes in each trimester, see our guide to the stages of pregnancy.
“Keepsake” ultrasounds and home Doppler devices
Obstetric ultrasounds are considered safe when performed for a medical reason. However, international scientific societies such as ISUOG recommend against long scans done only for keepsake photos or videos with no medical purpose. The principle is simple: as long as needed, no longer.
The same applies to handheld Doppler devices sold for home use. They can give false reassurance if the mother hears her own pulse, or if the baby has a problem despite a normal sound. If you are worried about your baby's movements, the right step is to contact your gynecologist the same day.
Frequently Asked Questions
How many ultrasounds are done during pregnancy?
There are three to four core scans: pregnancy confirmation, nuchal translucency at 11 to 14 weeks, level II anatomy at 20 to 24 weeks, and a growth scan with Doppler in the third trimester. High-risk pregnancies need more.
Is ultrasound safe for the baby?
Yes. It uses sound waves, not radiation. Decades of use have shown no harm to the fetus when it is performed for a medical reason.
When is the best time for a 4D ultrasound?
Between the 26th and 30th week, when the face has formed and there is still enough amniotic fluid for a clear image.
What is the difference between 2D, 3D and 4D ultrasound?
2D gives the two-dimensional black-and-white image used for medical diagnosis. 3D reconstructs a still three-dimensional image, and 4D shows it in real time, with movement.
When can you see the baby's sex?
Usually with confidence from the 16th to the 20th week, provided the baby's position allows it. See the full scope of prenatal screening.
Why does the due date change after the ultrasound?
The date calculated from your last period assumes a 28-day cycle and ovulation in the middle of the cycle, which is often not the case. Measuring the embryo in the first trimester is more accurate, so when the difference is significant, the date is corrected based on the ultrasound and is not changed again later.
How accurate is the ultrasound estimate of the baby's weight?
The estimated weight can differ from the actual weight by about 10 to 15%, either higher or lower. That is why decisions are based on the growth trend across successive scans and not on a single measurement.
What does a low-lying placenta mean at the 20-week ultrasound?
In most women, the placenta moves away from the cervix as the uterus grows. It is checked again around the 32nd week. Until then, if you have vaginal bleeding, contact your doctor immediately.
