Pregnancy Ultrasound Scans: What to Expect by Trimester

For many parents the ultrasound scan is the most memorable part of pregnancy: the first flicker of a heartbeat, the first look at a tiny profile. It is also a medical tool that checks how your baby is growing and whether the pregnancy is progressing as expected.

This guide walks through what a scan is, how it is usually spaced across the three trimesters, and how to prepare for each one. Exactly which scans you have depends on your health and your doctor’s plan.

What Is a Pregnancy Ultrasound?

An ultrasound uses high-frequency sound waves that are too high to hear. A handheld probe, called a transducer, is moved over gel on your skin; the echoes that bounce back are turned into a picture on screen. Sometimes the probe is placed inside the vagina to get a clearer view, which is common in early pregnancy.

According to MedlinePlus (US National Library of Medicine), ultrasound does not use ionising radiation like an X-ray, and it has not been linked to harm when trained professionals use it properly. Experts still advise using it when it gives useful medical information, rather than only for keepsakes.

First Trimester (Weeks 1 to 12)

A first scan may be done early if you have pain or bleeding, if your dates are uncertain, or if you have had pregnancy problems before. It can confirm that the pregnancy is inside the womb, count the babies and detect the heartbeat.

Many guidelines include a routine scan between about 11 and 14 weeks. The NHS calls it the dating scan. It measures the baby to estimate the due date, which may change the date worked out from your last period. In many places a nuchal translucency measurement, a fluid-filled space at the back of the baby’s neck, is added as part of screening for conditions such as Down’s syndrome, alongside a blood test.

Tip: Preparation differs between scan centres. Some ask you to drink water and come with a comfortable bladder, others do not. Ask when you book so that you are not left uncomfortable or sent away.

Second Trimester (Weeks 13 to 27)

The detailed mid-pregnancy scan is often called the anomaly or 20-week scan. The NHS offers it at around 18 to 21 weeks, and MedlinePlus describes a routine scan between weeks 18 and 22. The sonographer looks at the baby’s head, brain, face, heart, spine, abdomen, kidneys, arms and legs, and at the placenta, the amount of amniotic fluid and the baby’s position.

A scan usually takes 20 to 30 minutes, and longer if the baby is lying awkwardly. Do not worry if you are asked to come back or move around; this is common and does not usually mean a problem.

What the scan cannot do

No scan can find every condition. A normal result is reassuring but does not guarantee a healthy baby, and an unusual finding does not always mean something serious. If the sonographer sees something unclear, your doctor may advise a repeat scan or additional tests and will explain the next steps.

Third Trimester (Weeks 28 to 40)

A routine scan late in pregnancy is not offered to every woman. Growth scans are more often advised if you are expecting twins, if your baby seems smaller or larger than expected, if you have diabetes or high blood pressure, or if there were problems earlier. These scans can check the baby’s growth, the position (head down, breech or sideways), the fluid around the baby and the placenta’s position.

Doppler ultrasound, which shows blood flow, may be used when there is concern about how well the baby is being supplied. If you have a condition such as preeclampsia or gestational diabetes, expect closer scan-based monitoring.

Are Scans Compulsory? What About the Baby’s Sex?

Scans are recommended, not forced, and the NHS notes that you can choose not to have them. However, they give valuable information and most doctors strongly advise them. See our antenatal care guide for where scans fit in.

In India, the law is clear on one point. Under the Pre-Conception and Pre-Natal Diagnostic Techniques (PC&PNDT) Act, ultrasound clinics are not allowed to determine or reveal the sex of the foetus, and families should not ask. Please do not request or share this information. A scan is for your baby’s health.

Making the Most of Your Scan

• Bring your earlier reports and your antenatal card so that comparisons can be made
• Wear a loose, two-piece outfit so that your abdomen is easy to expose
• Write down questions beforehand, as it is easy to forget them
• Bring a support person if you wish, but ask the centre about its policy
• Ask for the report in writing and keep it in your antenatal file

Types of Scans You May Hear About

• Abdominal scan: the probe moves over the tummy, and it is the usual method from the second trimester onward
• Transvaginal scan: a slim probe is placed gently in the vagina, mainly in early pregnancy, to get a clearer picture. It is generally well tolerated and you can ask for a female sonographer or a chaperone if it helps you feel at ease
• Doppler scan: shows blood flow in the baby, the umbilical cord or the womb, and is used when there is a specific concern
• Growth scan: measures the baby’s size over time, so one measurement is compared with earlier ones

Understanding Your Report

Scan reports contain many abbreviations. Common ones include CRL (crown–rump length, used for early dating), BPD and HC (head measurements), AC (abdominal circumference), FL (femur length), EFW (estimated fetal weight) and AFI (amniotic fluid index). A figure that looks unusual on paper is not necessarily a problem. Measurements are estimates with some margin, and your obstetrician reads them alongside your examination and history.

Please avoid comparing your report with another mother’s. Babies grow at different rates, and a number that is normal for one pregnancy may mean something else for yours.

Twins and High-Risk Pregnancies

If you are expecting twins or have a condition such as diabetes, high blood pressure or a previous pregnancy problem, you will probably have more scans. They are not a sign that something is wrong; they are a way of watching closely so that any change is caught early. Our article on high-risk pregnancy explains what extra monitoring may involve.

When to Call or Visit

Do not wait for the next scheduled scan if you notice vaginal bleeding, severe or one-sided abdominal pain, leaking fluid, a marked drop in your baby’s movements, or fever with pain. Call your doctor or go to the hospital immediately.

For routine check-ups, you can book an appointment or call +91 944 103 0587. Chaitanya Hospital in Kadapa lists antenatal check-ups, including scans, among its services; see antenatal check-ups. Delivery and emergency care are open 24/7.

This article is for general education only. It is not a substitute for a doctor’s advice, diagnosis or treatment. Please talk to your own doctor about your situation.