Extension Education: Understanding and Reducing Early Pregnancy Loss in Ewes

By Dr. Whit Stewart and Micah Most

Many sheep producers have experienced the frustration of scanning a ewe flock in mid-pregnancy with high expectations, only to find later the lambing percentage doesn’t quite match the scanning results. Where do those “missing lambs” go? 

Research on fetal loss in sheep provides important lessons about management, nutrition and why what happens in the first trimester of pregnancy matters so much.

Ewe pregnancy

Sheep pregnancy can range from 142 to 152 days, progressing from the embryonic to fetal stages. 

The first 50 days – the embryonic stage – involve implantation and rapid development of vital systems like the nervous system, heart and limb buds, making this period especially vulnerable to stress, illness or poor nutrition. 

Around day 40, the pregnancy transitions to the fetal stage, when organs mature, the skeleton hardens and the fetus gains most of its weight. 

Because early losses are difficult to detect, consistent nutrition and minimal stress – especially in the first trimester – are critical to lamb survival.

Underlying these stages is a delicate cascade of hormonal and immune signals which sustain early pregnancy. 

In the first 12 to 15 days post-conception, the embryo secretes interferon tau (IFN-τ), a protein signal critical for maintaining pregnancy, which prevents prostaglandin F2α from triggering regression of the corpus luteum (CL). 

The CL, in turn, produces progesterone, a hormone essential for maintaining the uterine lining and embryo viability. Disruptions in this signaling due to stress, nutritional deficits or sudden environmental changes can lead to early embryo loss. 

Simultaneously, vascular endothelial growth factor (VEGF) promotes uterine and placental blood vessel development, supporting implantation and fetal nourishment.

The first 30 to 45 days of gestation are especially vulnerable. During this time, embryos must implant, establish a placenta and initiate stable hormonal communication with the ewe.

Fetal loss

Research shows 20 to 40 percent of potential lambs are lost in the first month of gestation, often before pregnancy can be detected. 

Many of these early losses are linked to embryonic developmental issues, including genetic abnormalities, chromosomal defects or hormonal imbalances in the ewe. 

Advanced tools like transrectal ultrasound confirm most losses occur before day 29, with some happening as early as day 14.

In real-world commercial production environments, pregnancy is typically assessed by ultrasound between days 45 and 90. This window offers the most accurate fetal counts available but only captures pregnancies which have already survived the high-risk early stages. 

Because commercial flocks often operate under variable nutritional and environmental conditions, scanning nearly always overestimates lambing outcomes – some fetuses visible at mid-gestation do not survive to term. 

After about day 60, pregnancies tend to stabilize and further loss becomes less common unless ewes face significant stress, disease or undernutrition. 

Supporting ewe health and minimizing stress throughout gestation – especially in the first trimester – helps reduce the gap between scanning percentages and actual lambing results.

Not every pregnancy loss means the ewe loses the whole litter. Many losses are partial, where one lamb is lost but others survive to birth. Research has shown this is fairly common. 

For example, more than one-third of ewes experiencing loss only lost a single fetus, while far fewer lost the entire litter.

More recent work reported about 18 percent of twin- or triplet-bearing ewes had partial litter loss, with survival often depending on the mix of males and females – litters with more male fetuses were more likely to lose one or more lambs before birth.

Other studies have shown some rams sire pregnancies where partial loss is more common, while others are more likely to sire pregnancies which either hold or lose the entire litter. 

Taken together, these findings show fetal loss in sheep often reduces litter size rather than causing complete pregnancy failure, which helps explain why a ewe may scan with twins but only lamb with a single.

Proactive approach

Managing fetal loss under extensive rangeland conditions requires a proactive and holistic approach. 

Adequate nutrition from the pre-breeding phase through the first 50 days of gestation is essential. 

Strategies like flushing – short-term supplementation before and during breeding – stimulate ovulation and increase the number of eggs released, helping to boost potential litter size. 

But sustained nutrition matters even more. Maintaining elevated nutritional levels during the first 45 days of pregnancy is critical, as abrupt decreases in feed quality or quantity can be more detrimental than a consistent shortage. 

In low-forage years, strategic supplementation such as increasing the feed allotted when pasture biomass is limited or rotating and stockpiling pastures to give ewes access to higher-quality forage helps prevent the energy and protein deficits which compromise early pregnancy.

Regularly tracking body condition scores is one of the most practical ways to assess whether ewes are maintaining adequate energy reserves during this time.

Second, handling ewes gently and minimizing stress during early gestation plays a critical role in embryo survival. Stress disrupts hormonal balance and immune function needed to support pregnancy. 

Producers should avoid unnecessary gathering, aggressive dogs or loud environments during breeding and early pregnancy. Vaccinations and some dewormers should be avoided during the first 45 days post-breeding to prevent unintended stress or harm to developing embryos.

Third, flock health must be monitored through vaccination, biosecurity and good hygiene.

Infectious agents like Chlamydia abortus and Toxoplasma gondii can cause mid- to late-term abortions and mummified fetuses. These diseases can spread quickly and silently in naïve or unvaccinated flocks, leading to sudden losses late in pregnancy. 

Following a veterinary-directed vaccination schedule and implementing a quarantine protocol for new or returning animals reduces this risk.

Key takeaways

Understanding the biology and vulnerabilities of early pregnancy is essential for bridging the gap between scanning and lambing. 

Strategic management decisions related to stress, nutrition and health interventions in early gestation can make a measurable difference in lamb survival and overall reproductive success. 

With thoughtful management, producers can narrow the gap between scanning and lambing and increase the number of live lambs on the ground come spring.

Dr. Whit Stewart is an associate professor and Extension sheep specialist at the University of Wyoming (UW) and can be reached at whit.stewart@uwyo.edu. Micah Most is an Extension educator based in Johnson County and an active member of the UW Extension Sheep Task Force.