Sexing of Embryos at the Time of Twin Reduction: A Clinical Approach

SIMPLE SUMMARY: In dairy cows, twin pregnancies and twinning are highly undesirable as they compromise health, welfare, and productive lifespans. We propose that the negative effects of twinning can be avoided by inducing twin reduction. Among dairy cows in their third lactation or more, the incidence of twin pregnancies may be as high as 30%, and different-sex twins represent around 50% of all twin pregnancies. This study provides new unique information on twin pairs during the late embryonic period (28–34 days of pregnancy) that allows for sex selection at the time of embryo reduction, such as differential ultrasound size measurements in heterosexual twins, different intrauterine embryonic growth patterns, and the different vulnerability of female and male embryos following the induced reduction in their co-twins. After heterosexual twin reduction, when the survival embryos were male, pregnancy loss was null. ABSTRACT: This study provides new unique information on bovine twin pairs during the late embryonic period (28–34 days of pregnancy) in relation to (1) a predictive ultrasound measurement that was differential for sexing heterosexual twins; (2) intrauterine embryonic growth patterns in twin pairs; and (3) a higher vulnerability of female embryos compared to males following an induced embryo reduction in heterosexual twins. The study population comprised 92 dairy cows carrying bilateral twins. A length difference between co-twins equal to or greater than 25% in around 50% of pregnancies served to determine the sex of embryos with 100% accuracy in heterosexual twins, which was assessed four weeks later on the remaining fetus after twin reduction. The apparent rates of growth of twin pairs and of individual male and female embryos from day 28 to 34 of gestation were similar to established growth pattern standards for singletons. Mean embryo sizes in relation to gestational age were smaller by some 5 days’ growth equivalent in twins compared to singletons. After the reduction in the female embryo in heterosexual twins, the risk of male embryo loss was null. This new information allowed for sex selection at the time of twin reduction.