This week I cover a topic that it is fair to say has shaped the last few years of my life – namely, equine pregnancy. It is particularly topical given the launch of the new iScan which is perfect for advanced equine pregnancy work. Although previous articles have touched on this subject where I discussed my PhD in general, here I will discuss particular landmarks you should look out for during early equine pregnancy (day 120 of gestation).

Horses have an average gestation period of 340 days, although this can vary. Following ovulation (which will be termed day 0), and subsequent fertilisation, the equine embryo begins its journey by gaining access to the uterus between days 6-7 of gestation. The embryo then continues to move throughout the uterus before ‘settling down’ at the bottom of one of the uterine horns between days 16-17 of gestation. This continuous movement of the embryo is a very unusual feature of equine pregnancy that is currently not well understood. Day 35 of gestation marks the completion of essential organogenesis of the embryo, which can now be termed a foetus.

A fascinating and unique hallmark of equine pregnancy is the development of the ‘endometrial cups’ (so-called due to their cup-like appearance).  Endometrial cups start their life as a band of foetally-derived trophoblast cells that stretch across the middle of the equine embryo from around day 24 of gestation, otherwise known as the chorionic girdle. These chorionic girdle cells then start to rapidly grow, become more specialised and break into the endometrium of the uterus between days 36-38 of gestation. They then round up and form endometrial cups, which can be found in a variety of different shapes and sizes. Endometrial cups are formed around day 40 of gestation.

It should be noted that the extent of cell invasion is one way by which reproductive biologists draw distinctions between different ‘types’ of pregnancy. By way of an example, human pregnancy displays intense invasion of the fetal cells into the maternal bloodstream, whereas as a general rule, horse pregnancy does not display such extreme cell invasion – with the exception of the endometrial cups. One of the main jobs of the endometrial cups is to secrete the pregnancy hormone chorionic gonadotrophin (eCG), which plays a variety of roles during equine pregnancy.

Finally, day 120 of equine gestation marks endometrial cup cell destruction and death, and can also be considered as the completion of early equine pregnancy (above information taken from the thesis of Robbin, M.G. 2012, and Allen and Wilsher Placenta 2009 and Allen, Reproduction 2000). It should be noted that there are many other aspects of equine pregnancy that have not been able to be covered here, and will be discussed in future articles.