The Liley plot has been modified by several author to handle pregnancies that are less than 27 weeks gestation.
One modification of Liley's work was used by the American College of Obstetricians and Gynecologists. This modification extrapolates the Liley diagram to 24 weeks of gestation.
A second modification extends the diagram to 16 weeks of gestation. This consists of 2 lines bent to give a peak maximum at 24 weeks gestation. These divide the diagram into 3 zones.
Zone
Significance
1
not anemic
2
affected and anemic
3
intrauterine demise possible if not treated
Line
Significance
delta OD 450
16 Weeks
24 Weeks
36 Weeks
A
separate Zones 1 & 2
0.0418
0.087
0.0285
B
separate Zones 2 & 3
0.178
0.339
0.119
where:
• The calculation of the delta OD450 is discussed in the previous 2 sections.
