No. Effacement is based on the length of the cervix. A 50% effaced cervix is halfway to 0%, which is the amount needed to give birth. Effacement and dilation are rarely an indication of impending labor, unless they're happening rapidly or are accompanied by other symptoms such as labor pains. Some women begin to efface and dilate weeks or months before labor, while others may not have any effacement or dilation at all until labor begins. At 50% effaced, the baby could be weeks away, or days; there's no way to tell.