Obstetric Ultrasound Examinations

Obstetric ultrasound uses sound waves to produce pictures of a baby (embryo or fetus) within a pregnant woman, as well as the mother's uterus and ovaries. It does not use ionizing radiation, has no known harmful effects, and is the preferred method for monitoring pregnant women and their unborn babies.

The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that pregnant women have routine obstetric ultrasounds between 18 weeks and 22 weeks gestational age in order to confirm pregnancy timing, to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, and to assess for congenital malformations and multiple gestations (i.e. twins) Additionally, the ISUOG recommends that pregnant women have obstetric ultrasounds between 11 weeks and 13 weeks 6 days gestational age in countries with resources to perform them. Performing an ultrasound at this early stage of pregnancy can more accurately confirm the timing of the pregnancy and can also assess for multiple fetuses and major congenital abnormalities at an earlier stage.

Early First Trimester: < 12 weeks

Imaging at this early stage is often by transvaginal scan as views are often not clear enough transabdominally
Reasons for an early scan:

Uncertain of dates
Vaginal bleeding
Abdominal pain
Past history of miscarriage or ectopic pregnancy
Excessive nausea

Information that can be gained from the ultrasound:

Check that the embryo/fetus is alive (heart beat visible)
Calculate the estimated due date by size of embryo/fetus
Diagnose multiple pregnancy, ectopic pregnancy or miscarriage
Check for uterine fibroids or ovarian cysts.

First Trimester: 11-14 weeks

To check that pregnancy is viable and ongoing
Transabdominal scan +/- transvaginal
To confirm pregnancy due date by checking size (normally 45 – 84mm from head to bottom or crown rump length)
Early review of the fetal anatomy but detail is limited due to small size (skull, brain, upper and lower limbs, heart, stomach, cord insertion)
To check for markers for Down Syndrome (nuchal translucency & nasal bone)

Second Trimester Scan: 18-22 weeks

Transabdominal +/- transvaginal
Purpose of the ultrasound:
To check fetal size and confirm pregnancy due date
To check for abnormalities of the fetal anatomy (The anatomy review includes the face, skull, brain, spine, heart and outflow tracts, lungs, diaphragm, stomach, kidneys, bladder, cord insertion, 3 vessel cord, upper and lower limbs and extremeties)
To check placental location
To check that there is a normal amount of amniotic fluid
To check the cervical length is normal
To check for presence of uterine fibroids or ovarian cysts
Fetal gender can usually be assessed on patient request
If you have any concerns about a particular abnormality (eg. Previous pregnancy of family history of an abnormality,) please advise the doctor prior to the scan.
Limitations of the ultrasound:
For good views of the fetus the ultrasound waves need to travel a short distance between the fetus and probe. Therefore, if fetal position is not good or there is thicker abdominal wall due to increased maternal weight views may be suboptimal. If fetal position is an issue, then a repeat scan later in the day or within a week is usually necessary to complete anatomy review. If maternal weight is an issue a repeat scan in a few weeks’ time when the fetus is larger may be necessary.
While most major abnormalities can be detected by ultrasound, not all abnormalities can be detected by second trimester ultrasound. There are many reasons for this including
The problem may not be present at the time of the scan
The problem does not produce changes that can be detected on ultrasound
Some abnormalities are very small and beyond the resolution of ultrasound

Third Trimester Scan: 23+ weeks

Transabdominal +/- Trasnsvaginal
Not every patient will need a third trimester scan
There are many reasons for having a third trimester scan
Measuring small or large for gestational age
Rechecking the placental location following a low-lying placenta at midtrimester scan
Vaginal bleeding
Maternal medical condition that may affect fetal growth or wellbeing such as Diabetes
Uncertainty of fetal position or presentation
Reduced fetal movement
Review of fetal abnormality
Information that can be gained from a third trimester scan
Estimation of fetal size (accurate to 10-15% difference)
Fetal position and presentation
Placental location
Amniotic fluid
Fetal wellbeing
Review of the fetal anatomy (views of some of the parts reviewed earlier in the second trimester may not be possible due to fetal size, position and crowding at this advanced gestational age)
Checking fibroids or ovarian cysts