Different Types of Antenatal Scans: A Comprehensive Guide for Expectant Parents

Why are antenatal scans done?

Antenatal scans use ultrasound, sound waves, to build a picture of the baby, the placenta and the womb. Across the pregnancy they confirm the pregnancy is healthy and in the right place, fix an accurate due date, count the babies, screen for chromosomal and structural problems, and track growth, fluid and blood flow. Each scan is timed to the window when its answer is most reliable, which is why the dates matter.

What are the main types of antenatal scans, and when are they done?

The table below is the usual sequence. Your obstetrician may add scans if something needs a closer look, or if the pregnancy is high risk.

Dating and viability scan (6 to 9 weeks)

This first scan is often done through the vagina (transvaginal) because the pregnancy is tiny and the probe gets a clearer picture; it is not painful and does not harm the baby. It confirms the pregnancy sac is inside the womb, which rules out an ectopic pregnancy, shows the heartbeat, counts the babies, and measures the baby from head to bottom (the crown-rump length) to fix the due date. This is the most accurate dating you will get; later scans are compared against it.

If you have bleeding or pain, the scan may be brought forward to 5 or 6 weeks. Our early pregnancy ultrasound page explains what is visible week by week and why a repeat scan is sometimes needed a week or two later.

NT scan and double marker test (11 to 14 weeks)

The nuchal translucency (NT) scan measures the small pocket of fluid at the back of the baby's neck, checks the nasal bone and looks at the early anatomy. It has to be done in a narrow window, ideally between 11 weeks 2 days and 13 weeks 6 days, so book it as soon as your dating scan gives you the dates.

On its own the NT measurement is only part of the picture. It is combined with your age and the double marker blood test to give one risk figure for Down syndrome and two other chromosomal conditions. This is screening, not diagnosis: a 'high risk' result means further testing is offered, usually a cell-free DNA blood test (NIPT) or a diagnostic test, and a 'low risk' result is reassuring but not a guarantee. Our guide to the double marker test and how to read the result explains more. In some women the doctor also checks blood flow in the uterine arteries at this visit, to gauge the risk of blood pressure problems later.

Anomaly or TIFFA scan (18 to 22 weeks)

The anomaly scan, also called the TIFFA scan (Targeted Imaging for Fetal Anomalies), is the most detailed scan of the pregnancy and takes 30 to 45 minutes. The sonologist works through the baby's brain, face and lip, spine, heart chambers and outflow vessels, stomach, kidneys, abdominal wall, arms and legs, and also checks the placenta's position, the amount of fluid and sometimes the length of the cervix.

Two things to know beforehand. No scan sees everything: some conditions are not visible on ultrasound and a few only show up later, so a normal anomaly scan is reassuring rather than absolute. And in India the law does not permit the baby's sex to be revealed at any scan, so the sonologist will not tell you.

Growth scans and Doppler (28 weeks onward)

From the third trimester, growth scans measure the head, abdomen and thigh bone to estimate the baby's weight, check the amount of fluid, confirm the placenta is clear of the cervix and note whether the baby is head down. In a normal pregnancy one growth scan at 28 to 32 weeks and another at 34 to 36 weeks is typical.

A Doppler scan in pregnancy is added when the doctor needs to see how well blood is flowing through the umbilical cord and the baby's own vessels: when growth is slower than expected, in high blood pressure or diabetes, if you have noticed fewer movements, or once the due date has passed. Our page on Doppler ultrasound and what it measures explains the readings. Women with a high-risk pregnancy are scanned more often, sometimes every 2 to 4 weeks, so growth and blood flow are tracked closely.

Are 3D and 4D scans necessary?

3D scans build a still surface image; 4D adds movement, so you see the baby yawn or move a hand. For most families they are an optional keepsake, usually clearest between 26 and 30 weeks; they do not make the medical checks any better. They do have medical uses, such as a closer look at the face, spine or a suspected cleft, and the same 3D/4D ultrasound technology is used in gynaecology to map the womb. If a 3D view is medically needed, your doctor will say so.

How do you prepare, and is ultrasound safe?

• Early scans through the tummy need a comfortably full bladder: drink 2 to 3 glasses of water an hour before and do not empty it. Transvaginal scans and scans after 20 weeks need no preparation.
• No fasting is needed for any pregnancy scan; eat normally, and a light snack before a growth scan can wake the baby.
• Wear loose two-piece clothing and carry your previous reports and antenatal card.
• Allow time: the anomaly scan is long, and any scan may need a second try if the baby's position is awkward.

Ultrasound uses sound waves, not X-rays or radiation. It has been used in pregnancy for decades with a strong safety record when performed by trained staff for medical reasons and kept as short as needed. Doctors avoid unnecessary Doppler in the first trimester and discourage non-medical 'souvenir' scanning, but you should not fear the scans your obstetrician has ordered.

At Unittas, the pregnancy scan centre in Tambaram does dating, NT, anomaly and growth scans on site, and our maternity packages bundle the scheduled scans with the antenatal visits. If you are early in pregnancy and unsure where to start, the pregnancy care team can set out the whole schedule at your first visit.

Each antenatal scan has a job, and together they let your obstetrician catch problems early and reassure you when all is well. Book them in their windows, ask questions during the scan, and remember that a scan can be brought forward whenever something worries you: bleeding, pain, a fluid leak or fewer movements never waits for the next appointment.