I am not familiar with the book mentioned by OP, although I have Hamaker-Zondag's book on Yods and unaspected planets, which is a wonderful work.
I do question the `planets in the 12th house' theory of in utero experience. However, I have noticed, just from my own rather random research, that any planets *close* to the Asc - either very late in 12th, or very early in 1st - do describe conditions either pre-birth, or immediately post-birth. For instance, having Neptune conjunct Asc on a natal chart appears to describe a health limitation, which is generally present for no more than 12-18 months of the baby's life. To give just one example, my grandson's half-sister was born with Neptune 31' short of her Aqua Ascendant. For the first year of her life she was in and out of hospital with a condition which took some time to diagnose, but was as a result of some muscles not yet being fully ready to function. By the time she was 15 months she was healthy. She has just turned 3. With natal Chiron very early in Pisces, and a little less that 3 degr past her Asc, I suspect this experience will have a lasting resonance for her, even if only at an unconscious level.
(At the time the child was having regular hospital visits, I detected almost an enjoyment in the adults - parents & grandparents - around her. She seemed to be used as an attention-getter. I'm hoping the little girl doesn't use illness to seek attention in her future life, but it is a possibility.)