Women who suffer from frequent headaches during pregnancy but have no history of them could have an underlying health condition, researchers have found.
Doctors examined the medical records of 140 pregnant women in New York who reported having headaches.
Of the 140 women, 65% of their headaches were migraines, however half of the other 35% were the result of pregnancy conditions involving high blood pressure, (mostly preeclampsia).
As a result, one in six women with a headache was diagnosed with a hypertensive disorder of pregnancy.
Dr Helen Webberley, GP for Oxford Online Pharmacy told HuffPost UK Parents: "Headaches are a very common symptom among pregnant women.
"Working out whether it is serious or not can be very difficult for the doctor and the pregnant woman alike."
The study was published in the journal Neurology.
The two most important factors that distinguished migraines from headaches with a secondary condition were high blood pressure or not having a history of headaches.
Women who didn't typically get headaches until they were pregnant were five times more likely to have a hypertensive disorder of pregnancy than a migraine.
Lead author Dr Matthew Robbins, said according to the Daily Mail: "Our study suggests that physicians should pay close attention when a pregnant woman presents a severe headache, especially if she has elevated blood pressure or lack of past headache history."
Dr Webberley says there can be many potential causes of headaches in pregnant women: "Stress, lack of sleep, worry about the pregnancy itself and childbirth, raising a young family, combining work and motherhood are all issues which can cause tension headaches.
"Dehydration can also play a part, as the baby is using what the mother consumes in order to grow and often expectant mothers do not take this in to consideration. Migraine sufferers can also find that their symptoms worsen during pregnancy.
"There are also the very serious headaches, such as those caused by high blood pressure and even pre-eclampsia.
"Although rare, pre-eclampsia can be very dangerous for the mother and baby and needs careful monitoring, and possibly early delivery.
"Pregnant women suffering from headaches shouldn't ignore them, but should ask their midwife to do a blood pressure and urine check.
"Any headaches considered to be 'worse' or 'different' to what they normally experience need to be checked out as soon as possible."