|
ABNORMALITIES IN THE SIZE OF THE GESTATIONAL
SAC |
- Normal sac size: Mean sac size(MSS) - CRL ³5mm.
- Small sac size : Mean sac size(MSS) - CRL <5mm.
- Normal sac growth = 1.13mm/day
- Abnormal gestations have a slower rate of sac growth = 0.7mm/day (1). This difference suggests that women with abnormally developing pregnancies in the first trimester have a slower rate of growth of the gestational sac than those with normally developing pregnancies have. This results in first trimester oligohydramnios.
- A sac small for the CRL of the fetus is predictive of miscarriage in 94% of such cases between 6 and 9 weeks of gestation. (spontaneous abortion in the control population with normal sacs was 8% (2).
- Abnormally large gestational sac.
|
|
|
|
|
|
Early gestational sac growth parallels embryonic growth. Discriminatory sac size refers to the critical size of the gestational sac at which a yolk sac or fetal pole should be visualized. Abnormally large sacs are transvaginally:
- MSD >13mm without a yolk sac. This value has been recently challenged (4) by workers who describe 30 patients that had normal outcomes despite nonvisualization of the yolk sac at an 8-19mm mean sac diameter. A discriminatory value of 20mm has a 100% specificity and 100% positive predictive value.
- MSD >18mm without a fetal pole (>25mm transabdominally).
Small Gestational Sac:
REFERENCES |
- Nyberg DA, Mack LA, Laing FC et.al. Distinguishing normal from abnormal sac growth in early pregnancy. J Ultrasound Med 1987;6:23-26.
- Bromley B, Harlow BL, Laboda LA, Benacerraf BR. Small sac size in the first trimester: A predictor of poor fetal outcome. Radiology 1991;178:375-377.
- Howe RS, Isaacson KJ, Albert JL, Coutifaris CB. Embryonic heart rate in human pregnancy. J Ultrasound Med 1991;10:367.
- Rowling SE, Coleman BG, Langer JE et.al. First-trimester US parameters of failed pregnancy. Radiology 1997;203:211-217.