What to Expect During an Obstetric Ultrasound in Pregnancy

Hearing your baby's heartbeat for the first time or seeing their outline on the screen is one of the most special moments of pregnancy. The obstetric ultrasound is the most widely used imaging test during pregnancy: safe, radiation-free, and full of valuable information. This guide explains what will happen at each appointment.

What is it?

The obstetric ultrasound —also called a sonogram or prenatal ultrasound— is an imaging test that uses high-frequency sound waves to create real-time images of the baby, the placenta, the amniotic fluid, and the uterus. Sound waves are not radiation: they generate no X-rays and have no known effect on the mother's or the baby's body.

There are two main types:

• Abdominal ultrasound: the transducer glides over the abdomen with a conductive gel.
• Transvaginal ultrasound: used in the first weeks of pregnancy, when the uterus is still small and is better visualized from the inside. The transducer is thin, is covered with a sterile sheath, and is inserted gently; discomfort is minimal.

The type of ultrasound indicated at each stage of pregnancy is determined by the OB-GYN or the provider managing your prenatal care.

When is it ordered?

An obstetric ultrasound is always performed on the orders of the doctor or the provider managing your prenatal care. Over the course of the pregnancy it may be requested at different points depending on the goal:

• First trimester (approximately weeks 6–13): confirm an intrauterine pregnancy, listen for fetal cardiac activity, calculate the gestational age and the estimated due date, and evaluate early developmental structures.
• Second trimester (around weeks 18–22): this is the anatomy (morphology) ultrasound, which studies the baby's anatomy in detail, the position of the placenta, and the amount of amniotic fluid.
• Third trimester: assess fetal growth, the baby's position, and well-being before delivery.
• At any point if the doctor considers it necessary because of a specific situation in your pregnancy.

Interpreting the findings is always up to the doctor managing your prenatal care.

How to prepare

Preparation depends on the type of ultrasound and the stage of pregnancy:

• Abdominal ultrasound (second and third trimester): some centers ask that you have a partially full bladder (drink water before the test without urinating), especially in early stages. Follow the instructions from your doctor or the diagnostic center exactly, since directions can vary.
• Transvaginal ultrasound (first trimester): generally no special full-bladder preparation is required; the doctor or the center will let you know if there are any instructions.

In all cases:

• Wear comfortable, two-piece clothing to make it easy to access your abdomen.
• Fasting is not necessary for a routine obstetric ultrasound.
• Do not stop any medication without first checking with your doctor.
• If you have questions or concerns about the test, write them down to ask the OB-GYN or the person performing the ultrasound.

Step by step: what happens on the day of the test

When you arrive: The center's staff will verify your medical order and your information. You will be taken to the exam room, where you can lie down comfortably on an exam table.

The abdominal procedure: A clear gel, at room temperature or slightly cool, will be applied to your abdomen. This gel is needed so the sound waves travel without interruption between the skin and the transducer. The specialist will glide the transducer gently over your abdomen, making small movements to capture different angles. It doesn't hurt.

The transvaginal procedure (if applicable): It is performed with a sterile sheath and lubricating gel. Insertion is gentle and, although there may be some pressure, it should not be painful. If at any point you feel significant discomfort, tell the specialist.

The images: On the screen you will see real-time grayscale images. The specialist will take measurements and photos or clips of different structures. Depending on the baby's position, some images may be easier or harder to obtain. Sometimes you will be asked to change position to improve visualization.

The baby's heart: Hearing the fetal cardiac activity is often one of the most exciting moments. The specialist will show you what they are seeing; if you have questions about what appears on screen, you can ask them.

When it's over: The gel will be wiped off with a towel or tissue. The specialist will hand over the report and the printed images (or digital files, depending on the center). The final report will be interpreted by your OB-GYN at the next visit.

Risks and recovery

According to current medical knowledge, obstetric ultrasound using sound waves is safe for both the mother and the baby. It does not use ionizing radiation. It is the first-line imaging test during pregnancy precisely because of its safety profile.

There is no recovery period. You can resume your normal activities immediately.

Cost

At private ultrasound clinics and doctor's offices in Mexico, the cost of an obstetric ultrasound generally falls between $550 and $1,000 MXN, varying by the type of study (basic, anatomy scan, Doppler), the city, and the equipment used.

Through IMSS or ISSSTE (Mexico's public health systems for private-sector and government workers, respectively), the obstetric ultrasound is part of prenatal care and is performed at no additional cost to the insured member, following the institutional prenatal care schedule.

Some major medical insurance plans include prenatal ultrasounds in their maternity coverage; review your policy before the appointment.