Fetal Ultrasound Measurements in Pregnancy
Ultrasound
Obie Editorial Team
Fetal ultrasounds are essential tools for monitoring the growth and development of your baby. These measurements can detect any potential abnormalities early on. During your pregnancy, different ultrasound measurements can be taken to ensure everything is on track. Key measurements can include the crown-rump length (CRL), biparietal diameter (BPD), femur length (FL), head circumference (HC), occipitofrontal diameter (OFD), abdominal circumference (AC), and humerus length (HL). Additionally, they can help calculate the estimated fetal weight (EFW).
What Fetal Measurements can be Calculated During Pregnancy?
At babyMed, you'll find an extensive selection of ultrasound and sonogram tools and calculators. These tools provide critical insights into pregnancy structures and embryo measurements—eight essential features will be explored below.
Gestational Sac (GS)
The gestational sac (GS) is an early pregnancy structure that can be seen on ultrasound by about 6 weeks of gestation. This sac encloses both the embryo and the amniotic fluid, working to nourish and protect the developing baby. The GS is crucial for confirming the presence and health of the pregnancy, helping determine if it's intrauterine or ectopic. It can be measured to help date the pregnancy within about 5 days of accuracy.
Utilize our fetal growth calculator!
Yolk Sac
The yolk sac serves as a nourishment source for the embryo before the circulatory system and placenta are fully functioning. Visible on ultrasound between the embryo and gestational sac, its size and shape help assess the health of the pregnancy.
Fetal Pole
The fetal pole is an early structure that evolves into the embryo. Starting from the 6th week, it grows approximately 1 mm per day. By measuring this, you can estimate the pregnancy's gestational age by adding the fetal pole's length (in mm) to 6 weeks. A 5 mm pole equates to a gestational age of 6 weeks and 5 days.
Crown-Rump Length (CRL)
CRL measures the distance from the top of the embryo's head to its bottom. Measuring between 7-13 weeks, it accurately estimates gestational age, starting within 3-4 days of the last menstrual period. Once established, the due date should remain, even if later scans suggest changes. Check any deviations as growth-related rather than revising the due date.
Biparietal Diameter (BPD)
BPD measures the distance across the head and becomes relevant after 13 weeks. Starting around 2.4 cm, it reaches about 9.5 cm at term. Head size can vary among infants of the same weight; thus, dating late in pregnancy using this measurement alone may not be reliable.
Find out the fetal size with our fetal measurements calculator.
Head Circumference (HC)
HC measures after 13 weeks, providing insights into the fetus's head size.
Check your's with our head circumference calculator!
Femur Length (FL)
This measurement reflects the growth of the fetus's longest bone. Beginning at 1.5 cm around the 14th week, it extends to approximately 7.8 cm at term. Like BPD, earlier measurements offer more reliable dating. Check out our femur length calculator.
Abdominal Circumference (AC)
AC is an essential indicator in late pregnancy, reflecting fetal size and weight more than age. Regular measurements help track the baby's growth. Avoid using AC for fetal dating.
Estimating Fetal Weight (EFW)
You can estimate fetal weight using equations that incorporate BPD, FL, and AC. For example, a BPD of 9.0 cm and an AC of 30.0 cm might predict a weight of about 2.85 kg. Entering AC, BPD, HC, and FL into a calculator offers a range of estimates. Remember, even the most accurate methods have a 15% margin of error.
Ultrasound Measurements in Pregnancy: The table below shows typical mean measurements for each gestational age from 4 weeks onward.
References:
- Gestational age calculations start from the first day of the woman’s last menstrual period.
- Sources for measurements include various scientific studies.
