Ultrasound (Prenatal)

Our department provides routine and specialized obstetrical ultrasound services to expecting families across  British Columbia and the Yukon.

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Care provider FAQ


In British Columbia, health care providers usually date a pregnancy using the earliest ultrasound where the crown-rump length (CRL) measures 10mm or more. This follows PSBC Ultrasound standards and SOGC Guideline No 388.

If you know the exact conception date through timed ovulation induction or in vitro fertilization (IVF), providers may use that date instead of the ultrasound.

The pregnancy date from a BC Women's ultrasound may differ slightly from dates given by other ultrasound clinics. This can happen if clinics use different CRL charts for first-trimester measurements.

BC Women's currently uses the Robinson CRL chart (PDF).

You can calculate the estimated due date and gestational age for a specific date by using the PSBC Pregnancy Dating Tool, based on the ultrasound date and the CRL measurement.


 
 

Health care providers usually check for soft markers of aneuploidy during the detailed ultrasound, which takes place between 17 weeks and 22 weeks, 6 days of gestation. Providers may skip this assessment if:

  • The patient already had non-invasive prenatal testing (NIPT) earlier, or plans to have it
  • The patient already had genetic testing, such as amniocentesis or chorionic villus sampling
  • The ultrasound request clearly states that the patient declined soft-marker screening

Providers assess and report soft markersbased on PSBC guidelines. You can find a link to the PSBC Trisomy 21 risk calculator here.

Important update

As of November 16 2020 (PDF), providers routinely check the following markers during the detailed ultrasound for all patients, even if NIPT results are available:

  • Presence of the nasal bone
  • Nuchal fold thickness
  • Bright (echogenic) bowel
  • Renal pyelectasis (mild widening of the kidney pelvis)

 

Health care providers assess the fetal nuchal region during the 11- to 14-week ultrasound. This exam looks at early fetal development.

If the patient already had non-invasive prenatal testing (NIPT) or preimplantation genetic testing for aneuploidy (PGT-A), providers do not report the nuchal translucency (NT) measurement for aneuploidy risk calculation.

For more detailed guidance, please see the memo on NT assessment in pregnancies with NIPT or PGT-A (PDF).


 
 

Because appointment space is limited, we follow specific booking guidelines (PDF) for pregnancies affected by diabetes.

For more details, please review the explanatory notes in the previously shared memo: BC Women's booking ultrasound guidelines: diabetes (PDF).



Health care providers routinely check placental cord insertion during the detailed fetal anatomy ultrasound.

They record findings and recommendations by following our local Abnormal Placental Cord Insertion Guideline (PDF).


Health care providers may hear an irregular fetal heart rate during a routine check. This type of irregular heartbeat is common and usually harmless.

The memo on Ultrasound and MFM referrals for irregular fetal heart rate (PDF) explains the first steps care providers should take when they detect an irregular fetal heart rhythm.

 

BC Women's provides an ultrasound schedule and clear explanations for pregnancies with a low-lying placenta or placenta previa. You can find this information in the BC Women's ultrasound schedule and explanatory notes (PDF).‎

 

 

If an ultrasound shows a small fetal head measurement, such as a small head circumference (HC) or biparietal diameter (BPD), follow the referral pathway (PDF) for next steps in care.‎

 
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Specialized exams

The information below explains when to refer patients for specialized obstetrical  ultrasound exams.​

 

Health care providers follow PSBC guidelines to decide when to refer patients for a nuchal translucency (NT) exam.

Because appointment spaces are limited, we may decline some booking requests. If this happens, you can find other locations that offer NT exams here.

 

 

‎BC Women's Obstetrical Ultrasound Department offers early, comprehensive first-trimester anatomical screening (PDF). This service is part of a pilot clinic for some pregnant people whose fetus may have a higher risk of congenital differences, as well as for those with a body mass index (BMI) over 35.



 
 

Health care providers use endovaginal cervical length ultrasound to monitor patients who have a higher risk of preterm birth. Providers usually perform this exam between 16 and 24 weeks of pregnancy. They often repeat the exam over time.

How often providers repeat the scan (usually every 1 to 2 weeks) depends on the cervical length measurement and the patient's risk factors.

Providers may request an endovaginal (EV) cervical length assessment if a patient has:

  • A past preterm birth before 36 weeks of pregnancy
  • A previous LEEP or cone biopsy
  • A cervical length less than 25mm in the current pregnancy before 24 weeks and 6 days
  • Maternal history of a connective tissue condition, such as Ehlers-Danlos syndrome
  • Maternal history a uterine condition, such as a unicornuate uterus
Providers may also use this exam to confirm cerclage placement:
  • They perform one scan before 24 weeks and 6 days after the procedure
  • They repeat the scan only if there is a concern that the cerclage is not working

Providers may refer patients for other reasons, such as signs or symptoms of preterm labour. The BC Women's ultrasound reporting physician reviews and prioritizes these referrals.

 

This assessment takes place between 20 and 24 weeks of pregnancy for people carrying one baby who meet the eligibility criteria listed in the risk assessment form. Providers submit this form as part of the MFM EMMA clinic referral (PDF).

The Maternal Fetal Medicine (MFM) team reviews all EMMA referrals. When MFM decides it is appropriate deemed appropriate – or when the care provider requests it – MFM pay provide an ultrasound-based consult within the ultrasound report instead of scheduling a separate MFM clinic visit.

When a care provider requests an ultrasound-based EMMA consult:
  1. The provider must submit a referral through MFM to book the ultrasound
  2. The ultrasound report includes recommendations after the exam
  3. The referring care provider must review the report and recommendations with the patient

If the ultrasound shows abnormal or unexpected findings, the ultrasound team notifies the referring care provider. In these cases, MFM may see the patient for a consultation.


The ‎referral criteria for specialized fetal heart exams are listed in this PDF. When you submit a referral, please clearly state the reason for the exam on the referral form.

 

We refer patients to the Fetal Diagnostic Service (FDS) Clinic when a prenatal ultrasound shows a possible abnormality.

You can find the referral criteria for the FDS clinic here.

If an ultrasound finding does not fall under the FDS clinic criteria, we review and prioritize the referral based on the BC Women's ultrasound triage guidelines (PDF).

 

You can find BC Women's most up-to-date umbilical artery Doppler protocol in this PDF.

Health care providers routinely perform umbilical artery Doppler ultrasounds in the following situations:

  • When the baby's abdominal circumference measures below the 10th percentile for their gestational age, in singleton or multiple pregnancies
  • When the pregnancy involves pre-eclampsia or gestational hypertension
  • When Maternal Fetal Medicine (MFM) requests the study, such as for complex monochorionic twin pregnancies

For other reasons, an MFM physician reviews the referral and decides whether to perform the study.

 

Health care providers use middle cerebral artery (MCA) Doppler ultraound to screen for severe fetal anemia in pregnancies where the baby is at higher risk. Providers commonly use this test in the following situations:

  1. Kell alloimmunization, at any antibody level
  2. Anti-D or other significant antibody alloimmunization once antibody levels reach the critical threshold, based on Canadian Blood Services guidelines
  3. Fetal-maternal hemorrhage, confirmed by a positive Kleihauer test
  4. Screening for twin anemia-polycythemia sequence (TAPS) in select cases of monochorionic twins (PDF) 

Health care providers should refer patients to the MFM clinic when they identify a fetus at risk for severe anemia casued by alloimmunization.

Providers may also request MCA Doppler studies for other reasons. MFM will decide whether the test is appropriate.

An MFM physician reviews and triages all requests for middle cerebral artery Doppler exams.


Technical protocols
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Use the BC Women's fetal heart screening checklist to guide image collection and interpretation‎ (PDF).

 

 

View the BC Women's updated Umbilical Artery Doppler Protocol (2024) for current guidance (PDF).

 

 

See the BC Women's ultrasound schedule and explanatory notes for pregnancies with monochorionic twins (PDF).‎

 

 

Review follow-up recommendations for persistent left superior vena cava (PLSVC) (PDF).‎

 
 

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