There are so many studies showing prenatal and infant difference in brain structure by sex I wouldn't even know where to start. Even more that specifically study the effect of prenatal hormones non developing fetal brains.

In humans? Really? Can you link to some of these studies?

I know that many papers have been written about sex-linked differences in brain structures of males and females of different animal species in utero and in infancy, and based on this many have theorized that marked sex differences exist in human brains in utero and infancy. I also know a ton of papers have been written speculating about the effect of prenatal (sex) hormones on human brain development too.

But your claim that "there are so many studies showing ... I wouldn't even know where to start" suggests these theories have been proven in human beings time and again. Is this really true? If so, how has the research been done?

Wouldn't this sort of research involve a whole lot of invasive testing of human fetuses to determine and measure their hormone levels at various stages in their development, and to ascertain that the hormones are reaching/affecting the brain? How would such testing be ethical? What mother (and father) would agree to this?

CVS (which I've had) and amnio come with known risks that are small but still significant enough that no one has these tests without there being a legitimate, proven medical reason. These tests also do not involve the fetus directly: CVS involves taking a piece of the placenta, and amnio involves taking amnio fluid. What's more, such test are done one and only one time in a pregnancy.

How could it be justified to do invasive testing on fetuses in utero again and again - not to benefit the health of the fetus or mother, but so scientist could see if a theory might or might not be true?

Moreover, wouldn't such research additionally necessitate that the same fetuses and infants who've had their hormone levels tested through invasive procedures repeatedly in utero also be subjected to MRI brain scans again and again in utero and after birth to chart exactly how their brains are developing and changing?

What mother (and father) would agree to that?

I know that fetal MRIs now can be done, and are done - but this is only in cases where an ultrasound/sonogram has revealed that a fetus already has brain abnormalities, or genetic testing has shown the fetus is likely to develop brain abnormalities.

How would subjecting fetuses and and newborns to repeated MRIs to test a theory pass an ERB? What parents and doctors would consider this ethical and would be A-OK with it?

From the Society for Pediatric Radiology (notice that most of the studies referred to are on other animals, not humans; and none discuss cases where either animal or human fetuses have been given repeated MRIs during the course of development):

Fetal Safety

The initial safety issue is that of exposure of the fetus to the static magnetic field [1,2]. Numerous animal model studies are present in the literature. Some have shown general deleterious embryonic effects such as upon development of embryos, delay and reduction in hatching rates, fetal loss, decreased crown rump length (CRL); additional studies have demonstrated organ specific disturbances in the development of the chick cerebellar cortex and murine eyes [3-8]. Other animal studies have shown no such embryonic effects, including a study using human fetal lung fibroblasts [9]. There are no studies in the literature that have reproducibly proven a deleterious biologic effect by static magnetic fields on human tissue [2,10]. While in vivo studies are understandably lacking, Kanal et al, in a survey of nearly 1500 pregnant MRI staff, found no statistically significant adverse pregnancy outcomes including fertility with exposure up to 4.7T [11].

Another safety issue is that of exposure of the fetus to excitatory radiofrequency energy and its inherent potential for tissue heating. Inherent to this issue is the potential teratogenicity of fever to the developing fetus [12]. A parameter for safety that is used is the specific absorption rate (SAR), defined as a measure of the rate of energy absorption by the body when exposed to radiofrequency electromagnetic field. The FDA and other international agencies have published SAR limits for whole body and local body, but it remains uncertain as to if the fetus should be considered a part of the mother or a separate individual. In one study, use of T2 HASTE, the foundation of fetal imaging, produced no significant temperature increase in the fetal brain or amniotic fluid of a pig model. And in a pregnant model, in normal mode at 1.5 T and 3.0T, the calculated temperature increase and SAR limits were found to be within a safe range [13,14]. There is however a report in the literature using a similar pregnant model where using maternal SAR limits may not protect the fetus from overexposure [15].

Another common safety concern is that of the use of rapid time varying electromagnetic fields (MR gradients) raising two issues: exposure to the gradients themselves and the acoustic noise created by the gradients. Limited research is available on isolated exposure to gradients, none of which have shown adverse effects with in vitro cellular studies [16,17]. More significant literature exists regarding high levels of acoustic exposure. Recent literature suggests that high levels of repeat sound exposure during pregnancy may produce low birth weights, shorter gestation and hearing loss [18]. The FDA limits sound intensity to 140dB in the MRI suite. The exact degree and frequency of sound attenuation as a function of gestational age is not known. Some level of sound attenuation however does occur in utero with attenuation more significant at higher frequencies offering the fetus some protections [19-21]. Exposure of the fetus to MRI at 1.5T in the second and third trimester has not been associated with hearing abnormality in several studies [22-25].

And finally, numerous studies evaluating longer term outcomes have also demonstrated no abnormalities in functional outcomes and birth weight for children exposed to in utero MRI [11,22,23,25-27].

https://www.pedrad.org/Specialties/Fetal-Imaging/Fetal-MRI-General-Information#38201347-safety

Finally, can we really assume that research done on brain development in other animals such as chickens is applicable to humans? I thought the unusually large size of the human brain - and its distinctive aspects and "superior" capabilities - relative to our overall size compared to other animals was sorta key. And that the unusually large size of the human brain, and thus head, as it develops in utero is why human childbirth is so difficult and painful. Other female mammals don't go through the same arduous labor and birth that humans do, coz they don't have to pass through their cervixes, vaginas and labia such big-headed beings as we humans do.