We color "pregnant" hair frequently, something to consider is her vitamin regime- does she take prenatal vitamins regularly and do they contain additional iron? If they do your color may be off tone from the iron which deposits in the hair and when lightened it oxidizes to a lovely pastel salmon pink. It does wash out after a couple of times, but can be noticeable if the iron dosage is high.
Your client will be fine, her color will be in foils, so as long as your application is on, there is very little risk for your client. Actually it's a huge benefit because her hair will look great and it will make her feel better about herself. If you've ever been 8 months pregnant you know EXACTLY what I mean:lol:!!!