Daily Growth Rate of the Fetal Yolk Sac
The yolk sac grows approximately 0.4 mm per week (95% CI 0.3-0.5 mm) from 5 to 10 weeks' gestation, which translates to roughly 0.06 mm per day. 1
Growth Pattern and Timeline
The yolk sac demonstrates minimal growth during early pregnancy, with the most reliable data showing a weekly increment of 0.4 mm starting at 5 weeks' gestation and continuing until 10 weeks' gestation 1
Converting this to daily growth: 0.4 mm ÷ 7 days = approximately 0.057 mm per day (essentially 0.06 mm/day), though this rate is so small that weekly measurements are more clinically meaningful than daily tracking 1
The yolk sac typically measures less than 6 mm throughout its visible period, with the largest yolk sac in viable pregnancies documented at 8.1 mm 2, 3
Clinical Context for Measurement
The yolk sac is first visualized at approximately 5½ weeks gestational age when the gestational sac reaches a mean diameter >8 mm, appearing as a thin-walled, spherical structure with an anechoic center 2, 4
The minimal growth rate explains why the yolk sac becomes obscured by the growing amniotic sac rather than continuing to enlarge proportionally with the pregnancy 5
Serial measurements are more valuable than single measurements for predicting pregnancy outcomes, as the yolk sac can reliably detect pregnancies destined for loss as early as 6 weeks' gestation 1
Prognostic Implications of Size
Before 8 weeks' gestation, a larger-than-expected yolk sac is associated with higher risk of pregnancy loss (P ≤ 0.001), while after 8 weeks, a smaller-than-expected yolk sac carries higher risk (P < 0.005) 1
A relatively large yolk sac (>95% upper confidence limits) in an anembryonic gestation or a yolk sac that progressively regresses are specific indicators of pregnancy loss 3
Abnormal yolk sac morphology (irregular shape, echogenic spots, or bands) is more specific than size alone for predicting pregnancy loss when embryonic cardiac activity is present 3
Important Caveats
The extremely slow growth rate (0.06 mm/day) means that daily measurements are not clinically practical or reliable due to measurement variability inherent in ultrasound technology 1, 5
Weekly interval assessments are the standard approach rather than attempting to track daily changes, as the growth increment is below the resolution threshold for meaningful daily clinical decisions 1, 5
The yolk sac's compensatory capacity means it may be larger in smaller mothers, suggesting involvement in embryonic growth regulation that can be traced to fetal size at 24 weeks' gestation 6