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The first trimester of pregnancy lasts about 14 weeks. During this time, your doctor may send you for an early obstetrical ultrasound, sometimes called a dating ultrasound. For more accurate dating results, we recommend the earliest you receive one is at seven weeks.
A fetal heartbeat might be detected as early as six weeks, but that is not always the case. Waiting until seven weeks gives the technologist a better chance of detecting the heartbeat and confirming other details about your pregnancy.
This exam will first confirm the pregnancy. Then it will examine and measure the embryo, as well as count the number of embryos. It will also establish an estimated due date. This information will help your doctor with prenatal care planning.
To estimate the gestational age of the embryo and thus your due date, the sonographer will measure the embryo from top to bottom, recording the crown-rump-length (CRL). At seven weeks it’s usually about the size of a peanut and measures around 10 mm long. During your exam, the embryo’s well-being will also be assessed by seeing and documenting the heartbeat. Your uterus and some surrounding organs will also be examined.
Once the need for an ultrasound has been identified, your doctor’s office may book an appointment for you, or provide you with a number to call to book an appointment. You will also be given a requisition form and preparation instructions for the exam. Please remember to bring it to the appointment.
For an early obstetrical ultrasound, you will need to arrive for your exam with a full bladder. A full bladder helps the sonographer to see your cervix and gently adjusts the position of your uterus to give the clearest view of the embryo.
Once in the exam room you will be positioned by one of our compassionate and experienced sonographers. A warm, unscented, hypo-allergenic ultrasound gel will be applied to your abdomen, and the sonographer will move the transducer around the front and side of your abdomen to gather images. You may experience mild pressure while the sonographer takes the images.
In some cases, the sonographer may need to perform an endovaginal ultrasound to obtain additional or clearer images of certain structures. The sonographer will explain the procedure to make sure you understand and consent to the exam. You will be asked to empty your bladder for this scan and a small probe will be gently inserted into the vagina.
It’s very common for the sonographer to leave the room to consult with the radiologist or confirm the images at the end of the exam.
Watch the video: What happens during a pregnancy ultrasound?
Ultrasound imaging is non-invasive and doesn’t use radiation. It has been used to evaluate pregnancy for almost 70 years and there has been no evidence of harm to the patient, embryo, or fetus. However, it should only be performed when medically necessary.
In order, to clearly visualize the fetus and acquire accurate measurements, the sonographer may need to apply pressure to your abdomen. It may be temporarily uncomfortable but shouldn’t be painful.
Please let us know if you experience any discomfort during your exam. We do understand that, while these ultrasounds can offer reassurance about the health of the embryo, they can also cause anxiety.
Most pregnancies have a gestation period of approximately 40 weeks, and you may have between one and four ultrasounds during that time (or more if deemed necessary by a health care practitioner). These exams help monitor the health of you and the embryo or fetus and inform care decisions.
A nuchal translucency ultrasound may be ordered in the first trimester to help determine the baby’s risk of having one of several congenital abnormalities. During this exam, the sonographer will take several specific measurements of the skin thickness at the back of the neck. This exam is time sensitive and can be booked between 12 weeks and 13 weeks, 6 days.
During the second trimester, at 18-20 weeks pregnant, your doctor will likely request a 60-minute detailed obstetrical ultrasound. Sometimes called an anatomic ultrasound, this exam involves the sonographer taking many measurements of the fetus from head-to-toe to determine how well it’s growing. The sonographer will obtain images to view the development of the fetus’ brain, face, heart, spine, chest, major organs, arms, legs, feet, and hands.
The sonographer will also assess the location of the placenta, the vessels in the umbilical cord, the amount of amniotic fluid, your cervix and uterus, and possibly the ovaries and bladder for abnormalities. It’s also during this exam when the gender of the fetus may first be visible – if it’s in a good position.
Throughout the rest of your pregnancy you may have one or more follow-up ultrasounds, called a biophysical profile and growth ultrasound. The purpose of these exams is to monitor the fetus’ growth and well-being. The heart rate, breathing, movements, muscle tone, and amniotic fluid will be assessed, as well as fetal size. Sometimes the fetus grows bigger or smaller than expected, so physicians may request these ultrasounds to help ensure a healthy pregnancy.
Obstetrical ultrasounds generally take up to 60 minutes, but this may vary depending on the type of exam, how movement of the fetus, and its position. The sonographer will do her or his best to ensure your comfort while also acquiring accurate measurements and the best possible images.
Please note, sometimes the sonographer may not be able to acquire all the images during one appointment and you may be scheduled for a follow up appointment to complete the exam. This may happen due to the fetus’ position, incorrect dates, or very early gestation.
Imaging and measuring the fetus during any obstetrical ultrasound can be challenging and requires concentration, so the sonographer will usually show you images of the fetus towards the end of your exam. We can also save images on a complimentary USB, to share with family and friends. Please let your technologist know at the beginning of your exam if you would like one.
Mayfair Diagnostics has 13 locations across Calgary which provide ultrasound services, as well as one in Cochrane and one in Regina. For more information about our clinic locations and services, please visit our clinic location pages.
REFERENCES
Healthwise Staff (2023) “First-Trimester Examinations and Tests.” myhealth.alberta.ca. Accessed May 28, 2024.
Mayo Clinic Staff (2022) “Prenatal care: 1st trimester visits.” mayoclinic.org. Accessed May 28, 2024.
Radiological Society of North America (2023) “Obstetric Ultrasound.” radiologyinfo.org. Accessed May 28, 2024.
Society of Obstetricians and Gynaecologists of Canada (2024) “Routine ultrasound.” pregnancyinfo.ca. Accessed May 28, 2024.
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