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Progesterone and estradiol levels can also be used in monitoring the hormonal status of the foaling mare. In general progesterone and progestogens gradual rise from day 15–20 pre-foaling to parturition and estradiol levels decline.

PROGESTERONE AND ESTRADIOL LEVELS IN THE TYPICAL FOALING MARE:
Days Prior to Parturition	PROGESTERONE (ng/ml)	TOTAL ESTRADIOL (pg/ml)
15	4.0–5.2	2800–3300
10	7.8–9.8	2100–2600
9	9.8–13.0	1700–2000
8	12.0–16.0	1600–1800
7	18.0–23.0	900–1600
6	22.0–27.0	750–950
5	25.0–30.0	580–700
4	28.0–32.0	500–600
3	29.0–33.0	350–430
2	31.0–37.0	320–415
1	35.0–42.0	270–320

The following two charts are designed as guidelines for drawing the appropriate samples for mares in various stages of their reproductive cycle and after breeding. Additional tests may be indicated under specific clinical presentations and pending reproductive complications.

PRE-BREEDING PROGRAMS:
TEST	MAIDEN MARE	BARREN MARE	POST-FOALING
Cytology	+	+	+ (day 15)
Microbiology	+ interpret with cytology	+ interpret with cytology	+ interpret with cytology
Endometrial Biopsy	if indicated by cytology	with abnormal palpation; ultrasound and/or histopathology > 2 years barren or indicated by cytology	– not indicated unless unable to conceive
Progesterone	+	+	+

This table serves as a guideline for mares entering the breeding program. Clinical history of barren mares may support endometrial biopsy upon presentation.

PROGESTERONE/HORMONE MONITORING AND THE POST-BREEDING MARE:
DATE/DAY Post-Breeding	Additional/Procedures Hormone Assays	INTERPRETATION OF REPRODUCTIVE HORMONE RESULTS
February 1	Basic Equine Screen/CBC Thyroid 1 (T3, T4)	Progesterone > 2 ng/ml indicates luteal tissue present. Progesterone > 5 ng/ml active cycling mare with CL; prostaglandin therapy should be considered. Progesterone < 1 ng/ml indicates estrus or no ovarian activity.
Day 2 post		Progesterone > 2 ng/ml indicates ovulation.
Day 8–10		Progesterone >7 ng/ml supports double ovulation. Progesterone < 2 ng/ml consistent with early endometritis or early luteal insufficiency. Progesterone 3.5–7.0 considered within normal limits.
Day 17	+ ultrasound	Progesterone > 3.5 supports conception with adequate progesterone. Progesterone < 1 ng/ml indicates estrus with no conception. Progesterone 2–3.5 ng/ml indicates luteal insufficiency and need for progesterone supplementation.
Day 32–34	+ ultrasound	Progesterone >3.5 ng/ml supports conception with adequate progesterone. Progesterone 1–3.5 ng/ml supports luteal insufficiency. Progesterone < 1 ng/ml indicates fetal death or non-conception and estrus.
Day 60	+ PMSG; +/- ultrasound	Progesterone > 3.5 indicates no need for supplementation. Presence of PMSG indicates endometrial cup formation.
Day 150	+ Estradiol	Progesterone > 3.5 indicates adequate placental progesterone production. Estrogen > 150 pg/ml supports viable fetus; may extend up to 1800 pg/ml in some mares.

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