Under medical supervision, keto can be a safe strategy for the prevention or reversal of gestational diabetes . Other than for that particular purpose, we don't recommend keto either during pregnancy or breastfeeding. There's some evidence that lactation can put a woman at risk of ketoacidosis  It's especially not safe to use the keto diet to lose weight during pregnancy or while breastfeeding as fat cells may release toxins , some of which may cross the placenta. There aren't many studies examining how keto affects preganancy and a baby's development, but the one's carried out do seem promising . Nonetheless, it would be a good idea to speak to your OBGYN or midwife if you plan to go keto during pregnancy or breastfeeding .