No its definitely not semi permenant and does not wash out!
It doesn't even fade much over time like the chemical permenants do.
I hennaed my reddish hair for a few years to liven it up. Then as I started to go grey, the grey hairs go orange...
So I decided to stop using henna and have professional colours done. No hairdresser would touch me as it permenantly coats the hair and gives very unpredictable results when you put anything else on it.
I had no option but to spend a year growing it out, which after 6 months of feeling and looking an absolutel mess, meant having a very short hair cut.
Henna is not only permenant, but if you get fed up of it or decide to go back to 'normal' colouring after the baby is born, you may find that hairdressers won't touch it.
Try asking a hairdresser about henna, they will suck their teeth and look frightened.