Pregnancy Testing in the Mare

Careful fetal and placental monitoring from the very beginning of the pregnancy can greatly improve the mare’s ability to carry the foal to term.

1. Progestagen assays alone will not confirm pregnancy in mares. Progestagen assays will tell if the mare is producing enough of these hormones to maintain the pregnancy.

2. BET prefers to test for PMSG (pregnant mare serum gonadotropin) also known as eCG (equine chorionic gonadotropin), when mares are between 45 and 90 days pregnant. This test has approximately a 5% false negative rate and a 10% false positive rate. PMSG values above 1 I.U./mL are considered positive; however, normal values are between 10 and 100 I.U./mL. Low levels of PMSG reflect fetal rejection by the mare. Mares which are below 10 I.U. and pregnant may be at risk of losing the pregnancy unless maintained on progestin therapy. Normal PMSG concentrations are 10-100 IU/mL with a peak at approximately 60 days of gestation. Some donkeys & miniature donkeys may not produce PMSG at all. Thus, this test is not very reliable in donkeys.

3. From 110 days to term, total estrogens assays are used to confirm pregnancy and monitor fetal viability. This test includes Estrone Sulfate and is approximately 99% accurate. Total estrogens assays work best between 100 days and 2 weeks prior to foaling. After 110 days of gestation Total Estrogens levels should be over 200 pg/mL; between 150 and 300 days it should be over 1,000 pg/mL. Low estrogens concentrations reflect fetal distress. Values will be similar for miniature mares, but different for donkeys and mini donkeys.

 

Approximately 2 weeks before foaling, total Estrogens levels begin to fall rapidly to approximately 300 pg/mL at foaling.

Progestagens rise and Total Estrogens fall prior to parturition in the mare. The rapid rise in Progestagens is thought to be due to Corticoid secretion by the fetus. The typical relationship between Progestagens and Total Estrogens prior to foaling is shown below. Not all mares follow this pattern. Alkaloids in the fescue and other plants prevent the dramatic rise in the Progestagens. The mechanism of this action is unknown.

Days post ovulationng/mL
0Less than 1.0
21.0 – 3.0
5 – 124.0 – 10.0
15 – 183.0 – 10.0
213.0 – 10.0
25 – 302.5 – 10.0
454.0 – 10.0
604.0 – 15.0
80 – 3155.0 – 10.0
316 to term20.0 – 40.0+
Results are those used by BET Laboratories and were generated by radioimmunoassay (RIA), which measures progesterone and some of its major metabolites (progestagens).
DAYSPROGESTAGENS (ng/mL)TOTAL ESTROGENS (pg/mL)TOTAL T4 (ng/mL)
0 days post-ovulation<1.0 less than 20012.0-25.0
2 days post-ovulation1.0-3.0 less than 20012.0-25.0
5-904.0-10.0 less than 200
12.0-25.0
100-1104.0-10.0 less than 20012.0-25.0
110-1204.0-10.0 200-5009.0-25.0
120-1304.0-10.0 300-6009.0-25.0
130-1404.0-10.0 400-7009.0-25.0
140-1504.0-10.0 500-10009.0-25.0
150-3204.0-10.0 greater than 10007.0-25.0
320-3254.0-15.0 800-10007.0-25.0
326-3306.0-20.0 600-8007.0-25.0
331-33510.0-30.0 500-7007.0-25.0
336-34020.0-40.0+ 300-5007.0-25.0
340+30.0-40.0+ 200-4007.0-25.0
post foaling<20.0 less than 20012.0-25.0

NORMAL LEVELS OF TOTAL ESTROGENS DURING PREGNANCY IN THE MARE

Total Estrogens test is valid after 110 days of pregnancy and its levels reflects the viability of fetal-placental unit. Normal Total Estrogens in donkeys are approximately 20% lower than Normal Total Estrogens in horses.

Approximately 2 weeks before foaling, Total Estrogen levels fall rapidly to around 300 pg/mL.
DAYS OF PREGNANCY
TOTAL ESTROGENS
Mean concentrations
(pg/mL)
BARREN / OPEN2-200
90 - 110
205
110 - 120
356
120 - 130469
130 - 140857
140 - 1501,040
150 - 320GREATER THAN
1,000

MEAN CONCENTRATIONS OF PROGESTINS AND TOTAL ESTROGENS PRIOR TO BIRTH

DAYS OF PARTURITIONPROGESTERONE (ng/mL)Total Estrogens (pg/mL)
154.83300
108.22100
910.11700
814.11600
721.0900
625.0850
527.0600
429.0550
331.0400
235.0410
136.0300
Progestagens rise and Total Estrogens fall prior to parturition in the mare. The rapid rise in Progestagens is thought to be due to Corticoid secretion by the fetus. The typical relationship between Progestagens and Total Estrogens prior to foaling is shown below. Not all mares follow this pattern. Alkaloids in the fescue and other plants prevent the dramatic rise in the Progestagens. The mechanism of this action is unknown.