Headaches are annoying. If you are pregnant, they may also be frightening — headaches can, after all, point to life-threatening pregnancy complications, including preeclampsia. They can also simply be the result of caffeine withdrawal if you've given up coffee to keep your baby safe, or be caused by stress, changed sleep patterns, and even simply pregnancy hormones.
Your Hormones Are Quite Literally Giving You A Headache
Research suggests that factors that lead to hormonal changes in women — from starting your period to using hormonal contraceptives and entering the menopause — can influence the frequency and severity of headaches. More than half of women who suffer from migraines report that their migraines get worse while they're on their periods, for instance. A small minority of female migraine sufferers even experiences migraines only during their menstrual periods! The same can be said for tension-type headaches, though less research is available in that area. [1, 2]
Pregnancy is another such significant milestone that leads to hormonal changes, and the fact that estrogen levels are increased during pregnancy could directly be to blame for your headaches.
A small number of women who have never had migraines before will develop them during pregnancy, however, and the four to eight percent of women with a history of migraines whose headaches worsen during pregnancy are more likely to experience migraines with an aura.  An "aura" means that you're experiencing neurological symptoms alongside your migraine. They may be things like dizziness, light sensitivity, seeing "stars", or seeing zig-zag lines .
Are Your Sleeping Patterns To Blame For Headaches During Pregnancy?
Research shows that the majority of pregnant women — around 75 percent — experience a change in sleeping habits. Getting less sleep during pregnancy is extremely common, through women often actually sleep longer than they usually do during their first trimesters. Hormones are partially to blame for these changed sleep patterns during pregnancy, too; progesterone has been shown to have the ability to induce insomnia. Other causes of altered sleep patterns include needing the bathroom more often, more shallow breathing, nausea and vomiting, and physical discomfort caused by your growing baby and belly. 
Insomnia and its related stress and anxiety have been shown to be risk factors for both migraines and tension headaches , but the oversleeping associated with the first trimester of pregnancy is also known to increase your chances of ending up with a headache .
- Going to bed and waking up at the same time each day.
- Banning electronics from your bedroom.
- Establishing a relaxing bedtime routine, involving a shower, relaxing music, and some calming reading, for instance. This will help send your body the message that you're getting ready to go to sleep. Don't engage in potentially high-stress activities such as paying bills or answering work emails right before bed.
Could You Be Dehydrated?
Dehydration is another possible cause of headaches during pregnancy . Even if you make very sure to drink at least two liters of water a day, pregnancy nausea and vomiting can quickly cause you to lose fluids and lead to dehydration. This risk is especially great in women suffering from hyperemesis gravidarum, or "morning sickness from hell" in which most foods and beverages come right out again. If this applies to you, please talk to your OBGYN as soon as possible.
Could Your Headaches Be A Sign Preeclampsia, A Serious Pregnancy Complication?
Preeclampsia, a life-threatening pregnancy complication, is characterized by high blood pressure and protein in the urine. It typically sets in any time after the 20th week of pregnancy. If you have severe headaches that won't go away, this may be a sign of high blood pressure caused by preeclampsia .
Other preeclampsia symptoms include:
- Vision changes — your vision might be blurred, or you may be very sensitive to light
- Nausea and vomiting
- Pain in the upper abdomen
- Breathing difficulties
- Not peeing as often as before
- Edema (fluid retention)
Other Serious Causes Of Headaches During Pregnancy
(Severe) headaches during pregnancy can also point to rare medical emergencies you've probably never heard of before:
- Cerebral venous thrombosis: A blood clot in the venous sinuses of the brain that prevents blood from flowing out of the brain, causing a stroke. CVT is the cause of just 0.5 percent of strokes, but pregnancy increases its risk slightly.  Symptoms include headache, blurred vision, fainting, seizures, and coma.
- Pituitary apoplexy: An infarction of the pituitary gland, this condition can be fatal. Symptoms include severe headache, vomiting, altered consciousness, and extreme light sensitivity. 
- Subarachnoid hemorrhage: A ruptured aneurysm in the space around the brain, this condition causes a severe headache, possibly along with speech difficulties, a drooping eyelid, double vision, a stiff neck, seizures, light sensitivity, and nausea and vomiting. Pregnancy is a known risk factor. 
Headaches During Pregnancy: What Now?
Headaches are pretty common during pregnancy. Though you may be scared now you know that several life-threatening medical conditions cause headaches during pregnancy, the fact that you've also read that they come with tell-tale symptoms you cannot miss should put your mind at ease. Nonetheless, it's important to mention headache during pregnancy to your doctor, both to rule out serious conditions, and to discuss possible relief.
Dr Sasa Milosevic, SteadyHealth's "in-house physician", advises pregnant women to avoid using over-the-counter painkillers for their headaches unless their doctors say it's OK. He adds:
"Even herbal medicines should be used with caution, as they can also contain substances which penetrate placenta and enter the circulation of the embryo or fetus. Everything which includes systemic, or even topically applied medications should be checked with a medical professional first. Keep in mind that there are only a few drugs that have been placed in the category A for use during pregnancy (drugs with no known adverse reactions). Categories B, C, and D should be avoided unless they are absolutely necessary, and the drugs from category X are a big NO.
Also have in mind that clinical trials with pregnant women represent a huge ethical problem, and the amount of information retrieved from them is very limited. Therefore, even the current recommendations about the safety of using any substances during pregnancy should not be taken for granted."
What can you do to relieve benign headaches during pregnancy, then? Here are some tips:
- If you've identified headache triggers, such as being in a crowded, noisy public space, avoid your triggers if you can.
- Exercise regularly, particularly outdoors.
- Eat meals at set times.
- Practice good sleep hygiene including getting up and going to sleep at the same time each day, and sleeping at least seven hours a night if you can.
- Avoid stress to whatever extent you can.
- The advice to not use OTC painkillers without consulting your doctor doesn't mean you shouldn't use them at all — talk to your doctor and ask them whether using a painkiller is OK. Acetominophen (Paracetamol, Tylenol) is one of the safest painkillers for use in pregnancy.