Table 4 Comparison of the baseline demographic features and true natural cycle characteristics of patients at different serum P₄ percentiles on FET-1

Female age, years

38.0 (33.0–40.0)

37.0 (33.0–39.0)

35.5 (32.0–39.0)

34.5 (31.5–40.0)

38.0 (31.0–39.0)

0.870

Body mass index, kg/m²

25.7 (22.2–29.7)^a

25.3 (22.3–27.6)^a

23.2 (21.2–26.9)

22.4 (20.8–24.8)

21.2 (20.1–23.7)

0.001

Antral follicle count, n

15 (7.0–21.0)

13 (8.0–18.5)

15 (8.0–18.0)

15 (9.0–19.0)

17 (9.5–18.5)

0.809

Follicular phase length, days

13.0 (10.5–15.5)

13.5 (11.0–14.3)

12.0 (10.8–14.0)

13.0 (11.0–15.0)

13.0 (12.5–15.5)

0.202

Follicle diameter one day prior to ovulation, mm

18.8 (17.5–21.4)^b

18.6 (17.2–20.6)^b

18.8 (17.6–20.4)^b

19.4 (18.1–21.0)^b

20.6 (19.0–22.6)

0.012

Peak-E₂ level, pg/mL

275.5

(198.0–335.2)^a

260.0

(209.5–323.5)^a

276.0

(196.2–334.2)^a

350.0

(292.5–413.0)

393.0

(330.2–488.5)

< 0.001

Peak-LH level, IU/L

40.4 (36.1–53.3)

44.4 (31.7–65.4)

44.9 (35.2–57.9)

43.6 (30.8–57.7)

45.7 (37.1–55.3)

0.993

Follicular phase peak-P₄ level, ng/mL

0.8 (0.6–1.0)^c

0.8 (0.6–0.9)^c

0.9 (0.7–1.2)

1.0 (0.9–1.2)

1.2 (1.1–1.4)

< 0.001

Endometrial thickness one day prior to ovulation, mm

10.8 (9.4–12.4)

10.7 (9.2–12.0)

10.2 (9.0–11.5)

9.6 (8.7–11.3)

10.6 (9.8–11.7)

0.149

LUF, n (%)

8 (47%)^d

4 (15.4%)

6 (13.0%)

7 (9.7%)

0.0

0.001^#