Embryonic Mortality and Pregnancy Loss in Cattle: Timing, Diagnosis, and Parental Contributions

Embryonic mortality and pregnancy loss remain major constraints to reproductive efficiency in both beef and dairy cattle. Despite relatively high fertilization rates, substantial losses occur during early gestation, creating significant biological and economic consequences. Understanding when losses occur, how they can be detected, and why they happen is essential for improving herd reproductive performance. 

Critical Windows of Pregnancy Loss 

Cattle exhibit high fertilization success. By day 7 post-insemination, approximately 65–75% of inseminated females have a viable blastocyst. However, pregnancy losses escalate during the first month of gestation¹. 

The most vulnerable period occurs between days 8 and 28, when the embryo transitions from the oviduct to the uterus and undergoes elongation and maternal recognition of pregnancy. Failure of adequate interferon-tau (IFNT) signaling results in corpus luteum (CL) regression, decreased progesterone (P4), and pregnancy loss^2,3. 

During the first month, total embryonic mortality may reach 44–50% in both dairy and beef cattle^2,3. Dairy cows, particularly high-producing animals, appear especially susceptible during maternal recognition and early placentation. 

Late embryonic and early fetal losses (after day 28–30) occur less frequently but still account for 5–15% of pregnancies depending on production type^1,2,3. These losses often go undetected under field conditions. 

Pregnancy Diagnosis: Strengths and Limitations 

1. Manual Methods¹ 

Rectal palpation 

Reliable after 40–45 days of gestation but limited in detecting early embryonic viability. 

Transrectal ultrasonography (US) 

Allows pregnancy diagnosis as early as days 26–29 and enables heartbeat detection between days 28–32. It is considered the gold standard for pregnancy confirmation and early loss detection when performed by experienced technicians. 

Doppler ultrasonography 

Evaluates CL blood perfusion and can diagnose non-pregnant females as early as days 20–22 with >90% accuracy. However, false positives (15–20%) limit its standalone predictive value for pregnancy viability. 

2. Chemical Pregnancy-Specific Tests 

Pregnancy-Associated Glycoproteins (PAG) 

PAGs are placental proteins detectable in maternal circulation from days 22–24 onward. Lower circulating PAG concentrations around days 28–31 are consistently associated with increased risk of late embryonic mortality^1,3,4. 

While PAG testing can achieve high predictive accuracy for late embryonic loss under controlled conditions, variability in assays, antibody specificity, and timing reduces consistency in field application¹. 

3. Chemical Non–Pregnancy-Specific Tests¹ 

Progesterone (P4) 

Elevated P4 between days 18–24 indicates functional CL maintenance and probable pregnancy. However, persistent CLs and extended luteal phases contribute to false positives. Low P4 is more reliable for identifying non-pregnant cows than predicting pregnancy loss. 

Interferon-Stimulated Genes (ISG) 

ISG expression in peripheral blood leukocytes reflects IFNT signaling during maternal recognition. Accuracy ranges from 70–90% in research settings. However, viral infections and other interferon responses reduce specificity. 

Circulating microRNAs (miRNA) 

Emerging research suggests specific miRNAs may differentiate viable from failing pregnancies as early as days 17–24. Although promising, lack of standardized extraction and assay protocols currently limits field application. 

Maternal Contributions to Pregnancy Loss¹ 

Replacement Heifer Selection 

Reproductive tract scoring (RTS) and antral follicle count (AFC) improve prediction of lifetime fertility. High AFC is associated with improved endocrine function and embryo production. 

Endometrial gene expression differences between high- and sub-fertile heifers suggest that pregnancy losses between days 14–28 may stem from impaired conceptus–uterine interactions . 

Estrus Expression¹ 

Estrus expression at insemination strongly correlates with improved pregnancy rates, higher PAG concentrations, and reduced embryonic mortality. Activity monitors and estrus detection patches enhance detection accuracy and reproductive outcomes. 

Reproductive Tract Size¹ 

In lactating dairy cows, larger reproductive tract size has been associated with decreased fertility and increased pregnancy loss between days 30–120, potentially due to altered sperm transport or maternal recognition mechanisms. 

Paternal Contributions: The Underestimated Factor 

While female physiology dominates reproductive research, sire effects on pregnancy maintenance are increasingly recognized. Significant variation in pregnancy loss during the second month of gestation has been observed among service sires despite similar early conception rates¹. 

Evidence suggests paternal genetics contribute substantially to placental development. Variability in placentome formation and trophoblast development may explain sire-dependent differences in late embryonic and early fetal loss. 

Importantly, traditional sire conception rate (SCR) indices may not capture post-implantation pregnancy losses, indicating a need for expanded fertility evaluation metrics. 

Vet Pearls 

• Day 24–30 is a critical blind spot: Many embryonic losses occur after early diagnosis but before routine rechecks. 

• Combine diagnostics: Doppler US + PAG testing improves predictive accuracy over single methods. 

• Estrus matters: Strong estrus expression at AI predicts both establishment and maintenance of pregnancy. 

• Don’t ignore the sire: Evaluate pregnancy loss between day 30–60 when comparing sires—not just conception rate. 

• Selection is prevention: RTS, AFC, and estrus expression monitoring improve long-term herd fertility. 

Conclusion 

Embryonic mortality in cattle is multifactorial, with the greatest losses occurring during maternal recognition and early placentation. Although significant progress has been made in diagnostic technologies—including ultrasonography, Doppler assessment, PAG measurement, ISG expression, and emerging miRNA biomarkers—no single method fully predicts embryo viability under field conditions. Both maternal environment and paternal genetics substantially influence pregnancy maintenance, particularly during the second month of gestation. Future advances in integrated diagnostics and genetic evaluation will be essential to reduce pregnancy loss and improve reproductive efficiency in beef and dairy production systems. 

References  

1. Pohler KG, Reese ST, Franco GA, Oliveira Filho RV, Paiva R, Fernandez L, Melo GD, Vasconcelos JL, Cooke R, Poole RK. New approaches to diagnose and target reproductive failure in cattle. Animal Reproduction. 2020;17:e20200057. https://www.scielo.br/j/ar/a/DpxF9tzL4wWJD735JjJCjPk/?lang=en&format=html  

2. Wiltbank MC, Baez GM, Garcia-Guerra A, Toledo MZ, Monteiro PL, Melo LF, Ochoa JC, Santos JE, Sartori R. Pivotal periods for pregnancy loss during the first trimester of gestation in lactating dairy cows. Theriogenology. 2016 Jul 1;86(1):239-53. https://doi.org/10.1016/j.theriogenology.2016.04.037  

3. Reese ST, Pereira MH, Edwards JL, Vasconcelos JL, Pohler KG. Pregnancy diagnosis in cattle using pregnancy associated glycoprotein concentration in circulation at day 24 of gestation. Theriogenology. 2018 Jan 15;106:178-85. https://www.sciencedirect.com/science/article/abs/pii/S0093691X17304946?via%3Dihub  
4. Pohler KG, Peres RF, Green JA, Graff H, Martins T, Vasconcelos JL, Smith MF. Use of bovine pregnancy-associated glycoproteins to predict late embryonic mortality in postpartum Nelore beef cows. Theriogenology. 2016 Jun 1;85(9):1652-9. https://www.sciencedirect.com/science/article/abs/pii/S0093691X16000406?via%3Dihub