You will see it all over the web that ketosis is dangerous for the fetus, but you won't find evidence. I see three classes of the so-called evidence. There is evidence that if you have diabetic ketoacidosis it is not good for the fetus, but we know that you can't compare benign dietary ketosis to diabetic ketoacidosis. There is animal evidence that if you starve pregnant rats, which also produces ketosis, it is not good for the fetuses. The flaw in that reasoning should be obvious. Finally, there is one experiment where they sliced up the brains of rat fetuses and soaked them in ketones, and the brain cells survived but the slices stopped producing new brain cells. This is supposed to be evidence of ketosis causing retardation.
On the other hand, a fetal metabolism text will tell you that fetuses are naturally using ketones themselves before and immediately after birth. This study shows that a pregnant pigs who are put on a ketogenic diet have fetuses with "increased fetal brain weight, protein content, and cell size." This text book article says "During early pregnancy there is an increase in body fat accumulation, associated with both hyperphagia and increased lipogenesis. During late pregnancy there is an accelerated breakdown of fat depots, which plays a key role in fetal development. Besides using placental transferred fatty acids, the fetus benefits from two other products: glycerol and ketone bodies. Although glycerol crosses the placenta in small proportions, it is a preferential substrate for maternal gluconeogenesis, and maternal glucose is quantitatively the main substrate crossing the placenta. Enhanced ketogenesis under fasting conditions and the easy transfer of ketones to the fetus allow maternal ketone bodies to reach the fetus, where they can be used as fuels for oxidative metabolism as well as lipogenic substrates..." In this study they inject pregnant rats with ketones and show that the fetuses readily use them for fuel. Similar here with sheep. In this one they took embryos and soaked them in ketones which either had no effect or caused them to grow.
Emily Deans also wrote something relevant and telling recently
Let me put it this way. Breastmilk is high in fat. Newborns (should) spend a lot of time in ketosis, and are therefore ketoadapted. Being ketoadapted means that babies can more easily turn ketone bodies into acetyl-coA and into myelin. Ketosis helps babies construct and grow their brains. (Update - looked more into this specifically and it seems that babies are in mild ketosis, but very young babies seem to utilize lactate as a fuel in lieu of glucose also - some of these were rat studies, though - and the utilization of lactate also promotes the same use of acetyl-CoA and gives the neonates some of the advantages of ketoadaptation without being in heavy ketosis.)
Couple those with the fact that some cultures like the Inuit ate an all meat diet, and that we probably evolved on it, I am not concerned about it.