Calgary Baby Guide 2023-2024

Fetal Sex
The 18-22 wk. ultrasound is often the one looking at the appearance of the fetal genitalia, thereby
disclosing fetal sex if parents want to know. Accuracy of fetal sex determination by ultrasound varies
according to timing of the assessment, experience of the operator, and fetal position; in good
circumstances, accuracy is close to but never 100%. Another way to know fetal sex is using a test called
Non-Invasive Prenatal Screening (NIPS), which analyzes small fragments of fetal/placental DNA that are
within maternal blood. NIPS is not covered by public health funding but may be done from the first
trimester of pregnancy and has a high sensitivity for sex chromosomes and common trisomy detection.
Some people will also have confirmation of genetic sex with amniocentesis or chorionic villus sampling
(CVS), if they happen to have those tests for other medical reasons.
Third trimester ultrasounds
Between 22-42 wk., ultrasound can estimate the fetal weight by measuring the head, abdomen, and long
bones of the baby. Estimated fetal weight (EFW) is interpreted with the help of fetal growth curves to be
meaningful, as all babies grow a lot in the 3rd trimester and knowing there is an inherent error range of
+/- 10% in the estimation itself. Fetal wellbeing is often assessed using the “Biophysical Profile Score” or
BPP (Biophysical Profile). The BPP is a score of 8- 10 points that looks at amniotic fluid, fetal breathing
movements, tone, body movements and sometimes heart rate. A BPP of 10/10, 8/8, or 8/10 with normal
amniotic fluid volume is a normal score. This is a powerful predictor of normal fetal acid-base status and
associated with a minimal risk of fetal death within a week. This assessment is occasionally repeated at
intervals when close fetal surveillance is indicated.
Third trimester ultrasound will determine fetal presentation, placental location and may assess placental
function in high-risk pregnancies. Think of the placenta as the baby’s lunch box, feeding the baby 24/7:
we certainly want that lunch box to keep working well throughout pregnancy! Additionally, ultrasound in
the third trimester can help decide on the best location for delivery (home, rural, tertiary hospital), type of
delivery (vaginal or cesarian section), timing of delivery (pros and cons of continuing the pregnancy or
not), and in some instances, discover rare anomalies only visible with more advanced fetal development.
Not everyone needs a third trimester ultrasound; it is always best to ask your healthcare provider.
24 • www.modernmama.com | CALGARY | A RESOURCE GUIDE | 2023/2024 Edition