As your baby grows, pressure from your uterus slows blood flow in your lower body, with the potential to distend
veins anywhere below the waist, including your
groin. Like
varicose veins on your legs, varicosities in your vulva can cause pressure and throbbing.
This will resolv eafter the delivery.
ref:http://www.melodywarnick.com/clips/your_belowthebelt_guide_to_pre.