Amniotic fluid in the first trimester

Amniotic fluid in the first trimester has been estimated from weeks 7-12. Although the amniotic fluid index (AFI) is calculated in the second trimester, one can get an idea of whether the amount of amniotic fluid is too much or too little at an earlier time point.

The amniotic fluid volume is related to, but different than the mean sac diameter.  The mean sac diameter is the measurement of the gestational sac, which included the amniotic sac. Although the first trimester amniotic fluid volume is not currently one of the criteria for pregnancy failure, an abnormal amount of fluid may indicate a poor outcome.

A difference between mean gestational sac diameter and crown-rump length of <5 mm is suspicious for, but not diagnostic of early pregnancy failure.

Radiographic features

Ultrasound

Estimated amniotic fluid volume (mean +/- standard deviation)

• week 7: 1.0 ml +/- 1.4
• week 8: 2.9 ml +/- 1.4
• week 9: 7.2 ml +/- 4.0
• week 10: 24.7 ml +/- 9.5
• week 11: 45.4 ml +/- 12.6
• week 12: 80.6 ml +/- 12.3

This is different than the gestational sac volume

• week 7: 8.3 ml +/- 4.3
• week 8: 16.2 ml +/- 6.8
• week 9: 26.0 ml +/- 10.8
• week 10: 47.1 ml +/- 19.6
• week 11: 67.9 ml +/- 20.9
• week 12: 90.8 ml +/- 14.3