**6. Early embryonic death**

Embryonic death is the most important source of reproductive losses. During the first three weeks of pregnancy embryonic deaths occur by means of several factors. If embryonic deaths take place between the 24th and 50th days, it is called as late embryonic death [4, 13].

Even in healthy cows in the first three weeks of pregnancy, more than 25% of the em‐ bryos cannot continue its development. While fertilization rate in cows with first service is 90%, calving rate is about 50-60% [4, 14]. In a study associated with this topic, it is report‐ ed that calving rate is 70% after insemination and most of the 30% of embryo losses take place in between 6th and 18th days [34]. If embryonic death happens before 16th - 17th days, cows continue to show estrus within normal intervals. However, if embryonic death hap‐ pens after 16th - 17th days, returning back to estrus cycle takes longer and cycle interval be‐ comes irregular [4].

There are plenty of factors that cause embryonic death in cows. These are; endocrine, genet‐ ic, intrinsic and extrinsic environmental factors, climate, stress, age, insemination time, se‐ men quality, infectious agents, nutrition, chromosomal anomalies. Especially, abnormal progesterone and estrogen profiles cause embryonic deaths. Moreover, in high producing cows steroid metabolism is faster because of liver blood circulation increase. And this causes lower levels of progesterone in luteal period of estrus cycle [4, 35].
