First of all let me know how much prunes u r taking for its laxative effect..it has carcinogenic effect and adverse effect on fertility in very high dose..let me inform u that constipation is very common during pregnancy.simple measures like 4-5lt water per day, lots of green leafy vegetables and usual day to day activities instead of complete bed rest will keep u free of constipation.if it fails then u can go for psyllium (ispaghula) husk at bedtime.if still not controlled then consult ur obstetrician for other drug options.