Moms-to-be, better get your shut-eye… but not too much… while you can. Severe lack of slumber is as much a part of new mommyhood as diapers and breastfeeding, but recent research shows getting just the right amount of sleep before you give birth is crucial.

A new study found that pregnant women who sleep six hours per night or less during their first trimester have elevated blood pressure later in their pregnancy: a systolic pressure 3.72 points higher than those who sleep nine hours. On the flip side, those who dozed for ten hours had a blood pressure that was 4.21 points higher than the nine-hour sleepers.

The findings highlight concerns over preeclampsia, a malady some women develop during pregnancy, usually after their twentieth week. Preeclampsia is a dangerous condition of high blood pressure associated with swelling of the limbs. It can constrict blood flow to the baby and is one of the primary causes of premature birth. Preeclampsia can also be deadly for the mother if left untreated.

The study found that women who got fewer than five hours of sleep per night were almost ten times more likely to develop the dangerous condition.

Rest may be hard to come by when your stomach is the size of a watermelon, but experts offer a few words of advice. For a more satisfying siesta, make time to exercise moderately, eliminate or cut back on caffeine and keep your weight in check. Resist the temptation to nap during the day.

Scientists aren’t sure why snoozing is linked to blood pressure, but for now, nine hours seems to be the magic number when it comes to sleep.

So before baby arrives, sleep to your heart’s content… or at least until your alarm clock goes off after nine solid hours of slumber.