False Pregnancy

Learn how to recognize this condition in your mare.

In mares, 5 to 10 percent of estrous cycles end in false pregnancies. They’re common enough that you should know how to recognize them. Normally, a mare ovulates follicles one to two days before she goes out of heat. A corpus luteum, or “CL”, then develops and begins to produce progesterone. The CL of a nonpregnant mare produces progesterone for about 14 or 15 days during the luteal phase of the estrous cycle. At the end of that phase, the mare’s uterus must determine pregnancy, a process called “maternal recognition of pregnancy.” First, the embryo produces a chemical signal and, second, the uterus detects the signal. If the uterus doesn’t find that signal, the endometrial lining of the uterus releases prostaglandins that travel through the bloodstream to the ovary and destroy the CL. Progesterone levels drop rapidly, and the mare develops another follicle and returns to heat. This cycle repeats itself multiple times each breeding season in a nonpregnant mare. If an embryo is present, no prostaglandins are released, the CL is preserved, and more progesterone is produced. Progesterone is a key hormone for maintenance of pregnancy. It also prevents the mare from returning to heat.

More Progesterone

In some instances, a nonpregnant mare’s CL can produce progesterone beyond the normal two-week lifespan and a mare won’t return to estrus. The term for persistent progesterone production by a CL is pseudopregnancy, or false pregnancy.

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If a mare loses the embryo after “maternal recognition,” false pregnancy can result. Other causes of false pregnancy can be diestrous ovulations, severe uterine pathology, and potentially inadequate prostaglandin release. Mares have one chance per estrous cycle to determine their own pregnancy status and to “recycle” themselves if they are not pregnant. The CL will be spared if a mare becomes pregnant and stays pregnant past the time of maternal recognition of pregnancy (days 12 to 14, postovulation). Mares that lose their pregnancy after the window of opportunity for maternal recognition will retain their CL.

Check with ultrasound

False pregnancies are not uncommon, so don’t rely exclusively on teasing to determine whether a mare is pregnant. Mares that fail to return to estrus two to three weeks after going out of heat are not always in foal. An ultrasound examination 14 to 16 days after breeding can determine whether a mare is pregnant and detect the presence or absence of twins. A recheck at 25 to 35 days can verify whether the mare is still pregnant. Pseudopregnant mares will not return to heat as long as the CL is producing progesterone. The lifespan of a retained CL can be two to three months. If the condition isn’t diagnosed and treated the breeding season could be over. False pregnancy is easy to treat if recognized. A single dose of prostaglandins will destroy the retained CL, and the mare will return to estrus in three or four days. However, the mare must be examined by ultrasound before she receives prostaglandins to be sure she is not pregnant. Pregnant mares that are given prostaglandins will lose their pregnancies.

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