Thanks for the interesting article @Cattenberg
I have now learnt about hemochorial placenta and epitheliochorial placenta which I didn't know before! This explains the growth differences in horses.
"In horses, placentation is epitheliochorial and occurs over the entire surface of the endometrium. Thus, the nutritional supply to the fetus, which depends on the contact surface between the placenta and the endometrium, is governed by the size of the uterus and therefore by the mare's size. Based on this observation, the impact of early life events on intra-uterine and post-natal development of the foals was demonstrated using artificial insemination to cross large Shire horses with small Shetland ponies [24]. More recently, Allen and his colleagues used embryo transfers between small and large breeds of equidae (ponies and thoroughbreds) as a model for fetal programming, restricting or enhancing fetal growth by transferring thoroughbred embryos into pony mares and pony embryos into thoroughbred mares, respectively. Fetal development was related to maternal size, with the gross placental area, weight and microcotyledonary density being the primary operative control mechanisms [25]. Increased or restricted post-natal growth of foals born to between-breeds embryo transfers were also associated with altered blood pressure and response of catecholamine to acute stress [26] and altered pancreatic β cell function [27] in the immediate neonatal period (first week after birth)."
Of course there are many factors which influence fetal growth and health in humans, and these have been shown to have lasting effects on the baby right through to adulthood.
"Epidemiological studies in humans have linked early-life events with a range of pathologies in adulthood. The first evidence of this was provided by the Hertfordshire's cohort in which people who had a small birth weight (reflecting suboptimal fetal development) were at greater risk of developing coronary heart disease, hypertension or type II diabetes in later life [1]–[3]. Maternal nutrition was pointed out as the primary factor affecting fetal development: in investigations of individuals who were exposed in utero to the Dutch Famine during World War II. It was shown that they were prone to a higher risk of developing obesity, glucose intolerance, hypertension or cardiovascular diseases in adult life [4], [5]. Rapid post-natal catch-up growth was also shown to increase the risk of later obesity as a result of a mismatch between the restricted in utero conditions to which the fetus had adapted and post-natal abundance [6]. In contrast, excess birthweight also leads to adverse programming, with a U-shaped curve for increased risks [7]."
There are many reasons why wealthy westerners using impoverished women in eastern countries and places like Ukraine are objectionable. But even aside of the ethical issues I find it bizarre that they wouldn't be more concerned about the possible impact of the health of these women on the baby they claim to desire so much might be. Factors that might effect the health of their child lifelong.
Link to the article I took quotes from for anyone interested in horse surrogacy!
Enhanced or Reduced Fetal Growth Induced by Embryo Transfer into Smaller or Larger Breeds Alters Post-Natal Growth and Metabolism in Pre-Weaning Horses
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4090198/