Table 2.

Threshold effect analysis of serum P level (ng/mL) at the hCG day on the LBR.

Outcome	Live birth
Model I (linear)	aOR	95% CI	p value
Linear effect	0.812	(0.590, 1.116)	0.199
Model II (polyline)	aOR	95% CI	p value
Predicted threshold (K, P level, ng/mL)	1.0
Effect 1 (<K)	2.162	(0.789, 5.923)	0.134
Effect 2 (≥K)	0.558	(0.340, 0.916)	0.021
variability of effectiveness	0.258	(0.068, 0.974)	0.046
Logarithmic likelihood ratio test	0.044

P, progesterone; aOR, adjusted odds ratio; C, confidence interval; K, predicted threshold.

Adjust for: female age, male age, number of transferred embryos, type of transferred embryos and endometrial thickness.