Pre-eclampsia usually occurs in the second half of pregnancy and can cause serious complications including seizures and death.
Placenta accreta is when the placenta attaches too deeply into the wall of the uterus and remains attached after childbirth.
Kardashian West said in a post on her app that "doctors said that it wasn't safe for my, or the baby's, health to carry my own".
The 37-year-old said she and West decided to use a "gestational carrier", meaning the baby is "biologically mine and Kanye's".
She wrote: "Having a gestational carrier is definitely different, but anyone who says or thinks it's the easy way out is completely wrong.
"People assume it's better because you don't have to deal with the physical changes, pain or complications with delivery, but for me it was so hard to not carry my own child, especially after I carried North and Saint."
Their surrogate was discussed on reality show Keeping up with the Kardashians, but her identity was not revealed. Kardashian West said they had a "great" relationship and "trusted her completely".
"I'm so grateful for modern technology and that this is even possible. It's not for everyone, but I absolutely love my gestational carrier and this was the best experience I've ever had.
"Having a gestational carrier was so special for us and she made our dreams of expanding our family come true."
