Yolk Sac can reliably detect pregnancies destined for loss as early as 6 weeks, finds study

The yolk sac (YS) is the initial embryonic structure visible on ultrasound within the gestational sac (GS), typically detectable as early as 5 weeks of gestation, corresponding to 3 weeks of embryonic development, thereby confirming pregnancy. The YS serves as the main source of red blood cells and germinal stem cells for the embryo. The previously held belief that the YS primarily facilitates nutrient transfer to the embryo during the 3rd to 4th week of gestation has now been updated. Recent prospective cohort study aimed to establish a nomogram of yolk sac (YS) growth during the first trimester of pregnancy and its relationship to pregnancy outcomes. The researchers serially measured YS diameters using 2D ultrasound in 193 patients with singleton, twin, and triplet pregnancies from 5 to 11 weeks of gestation. The key findings were: 1. There was no difference in YS dimensions between singleton and multiple pregnancies. In ongoing pregnancies, the YS increased linearly by 0.4 mm per week from 5 to 10 weeks. 2. 45 out of the 238 total fetuses were lost in the first trimester. In pregnancies destined for loss, the YS was either smaller or larger than in ongoing pregnancies, with this abnormality detected as early as 6 weeks. 3. Up to 8 weeks of gestation, the risk of pregnancy loss was higher with a large YS. After 8 weeks, the risk was higher with a small YS. All pregnancies with a large YS were lost by 10 weeks, while some with small YS were lost beyond the first 10 weeks. 4. The researchers established thresholds for YS diameter that predicted pregnancy loss at different gestational ages. A large YS above the median was associated with higher loss risk up to 8 weeks, while a small YS below the median carried higher risk after 8 weeks.

Conclusion and Implications

In conclusion, this longitudinal study provides the first detailed nomogram of YS growth in normal and abnormal pregnancies. It demonstrates the YS can reliably detect pregnancies destined for loss as early as 6 weeks, with either a large or small YS predicting different patterns of pregnancy failure. These findings could aid clinical counseling and monitoring of early pregnancies.

Key Points

1. There was no difference in yolk sac (YS) dimensions between singleton and multiple pregnancies. The YS increased linearly by 0.4 mm per week from 5 to 10 weeks in ongoing pregnancies.

2. 45 out of the 238 total fetuses were lost in the first trimester. In pregnancies destined for loss, the YS was either smaller or larger than in ongoing pregnancies, with this abnormality detected as early as 6 weeks.

3. Up to 8 weeks of gestation, the risk of pregnancy loss was higher with a large YS. After 8 weeks, the risk was higher with a small YS. All pregnancies with a large YS were lost by 10 weeks, while some with small YS were lost beyond the first 10 weeks.

4. The researchers established thresholds for YS diameter that predicted pregnancy loss at different gestational ages. A large YS above the median was associated with higher loss risk up to 8 weeks, while a small YS below the median carried higher risk after 8 weeks.

5. This longitudinal study provides the first detailed nomogram of YS growth in normal and abnormal pregnancies.

6. The findings demonstrate the YS can reliably detect pregnancies destined for loss as early as 6 weeks, with either a large or small YS predicting different patterns of pregnancy failure. These findings could aid clinical counseling and monitoring of early pregnancies.

Reference -

L. Detti et al. (2019). Pilot Study Establishing A Nomogram Of Yolk Sac Growth During The First Trimester Of Pregnancy. *Journal Of Obstetrics And Gynaecology Research*, 46. https://doi.org/10.1111/jog.14173.