Recurrent pregnancy loss can be frustrating and demoralizing for women undergoing IVF. Not only is losing a much-anticipated and carefully planned pregnancy heartbreaking for the patient, but it can also take a toll on their partner or family.
The most common IVF questions surround future pregnancy success. However, pregnancy outcomes are still difficult to predict in a clinical setting. Clinicians can gain insight into IVF success rates by using ultrasound to measure key parameters like crown-rump length (CRL).
Can Crown-Rump Length Predict Fetal Loss?
Some preliminary answers can be formed based on first-trimester obstetric ultrasound results. This routine ultrasound exam assesses the overall uterus, gestational sac, yolk sac, fetal pole and fetal heart rate. Both the visual appearance and size of the fetus provide valuable information about the health and viability of the pregnancy.
A study published in the American Journal of Obstetrics and Gynecology found that in a singleton pregnancy, a fetal crown-rump length of less than the 10th percentile at 40 to 80 days gestation is associated with an increased risk of fetal loss. The majority of losses occurred prior to a gestational age of 10 weeks.
CRL measurement can be used to accurately date a pregnancy between 7 and 13 weeks gestation, according to research published by the Global Library of Women's Medicine. A fetal size discrepancy at this stage may indicate an increased risk of miscarriage. A CRL of less than 7 mm with no fetal cardiac activity is suspicious of pregnancy failure, the American College of Obstetricians and Gynecologists reports.
The crown-rump length of female fetuses has been shown to be significantly smaller than that of male fetuses. Fetal size is not affected by the height, age or parity of the mother.
All of these factors must be considered when using CRL to determine the viability of a first-trimester pregnancy.
How to Measure Crown-Rump Length With Ultrasound
Measuring crown-rump length via ultrasound requires meticulous care, especially when assessing IVF success. The first CRL measurements provide the gestational age (GA) of a fetus and establish a growth baseline that will affect the management and future ultrasound biometry of the pregnancy.
For a correct CRL measurement, keep the following features in mind:
- Magnification: The fetal pole fills most or all of the screen.
- Mid-sagittal plane: The crown of the head and rump are clearly seen. If the fetus is over 9 weeks GA, the profile and spine should also be visible.
- Neutral position: The fetus is not flexed or tightly curled, and is in a relaxed, neutral position. If the fetus is over 9 weeks GA, there should be fluid visible between the fetal chest and chin.
- Horizontal to beam: The fetus is 90 degrees to the ultrasound beam.
- Calipers are correctly placed: The center cross of the calipers is placed precisely on the outer skin border of the head and rump.
While the fetal pole is visible on ultrasound by 6 weeks GA, the Journal of Obstetrics and Gynaecology Canada reports that the most reliable measurements happen closer to 7 weeks GA after a CRL of 10 mm is reached.
Transabdominal or transvaginal ultrasound can be used for this measurement. However, transvaginal views offer significantly more detail, especially if the pregnancy is under 7 weeks GA. CRL measurements should always be taken a minimum of three times, and the results averaged to provide the most accurate final number.
New Insights for Early Pregnancy Loss
Research on CRL and pregnancy loss is still emerging, but what we do know can provide some insight into the odds of IVF success. Although other factors must be considered — fetal heart rate, appearance and size of the yolk sac, gestational sac and overall uterine conditions — understanding this assessment can help clinicians provide more answers to anxious IVF patients on the state of their pregnancy.
