Why blood pressure falls

The HSE gives the mechanism in one line: "hormone changes — this can cause your blood pressure to drop and less blood to get to your brain." The NHS describes the consequence: "Fainting happens if your brain is not getting enough blood and, therefore, not enough oxygen."

Both then name the situations that provoke it. The NHS: "You are most likely to feel faint if you stand up too quickly from a chair or out of a bath, but it can also happen when you are lying on your back."

The HSE lists the other contributors: overheating, low blood sugar — its advice is to "keep a snack with you and avoid skipping meals" — and low iron levels. It also notes that dizziness "is common in weeks 0 to 13 of pregnancy" and "is sometimes the first sign that you are pregnant".

What a low reading actually means

Blood pressure gets measured at every antenatal appointment, but the reason is not to find low readings. NICE's antenatal care guideline sets thresholds only in the other direction — it instructs clinicians to "urgently refer women with severe hypertension (blood pressure of 160/110 mmHg or higher) to secondary care to be seen on the same day".

There is no equivalent low threshold in NICE's antenatal guidance, and no treatment recommended for low blood pressure in an otherwise well pregnancy. A reading that looks alarmingly low next to the numbers you remember from before pregnancy is, in most cases, exactly what the midwife expected to see.

What gets acted on is not the number but the symptoms attached to it: how often you feel faint, whether you have actually fainted, and what else is going on. It is worth saying so out loud at your appointment rather than trying to interpret the figure yourself.

The position that makes it worse

Lying flat on your back in the second half of pregnancy reduces the blood returning to your heart, and can make you feel faint without your having moved at all. The NHS's advice is direct: "It's better not to lie flat on your back in later pregnancy or during labour. You should avoid going to sleep on your back after 28 weeks as it has been linked to a higher risk of stillbirth."

If you feel faint while lying on your back, the NHS's instruction is simply to turn onto your side. The HSE applies the same logic to helping someone else: roll them onto their left side "with their left knee pulled up towards their chest".

What to do in the moment

The HSE's list is the most practical published anywhere, and it includes things people get wrong:

Tell someone. First on its list, because they can support you and get help.
Sit and lower your head towards your knees, or lie on your side.
Take deep breaths and breathe out slowly.
Get someone to open a window, and loosen tight clothing.
Do not try to move from where you are. Walking to find a better spot is how people fall.
Do not eat or drink anything until the dizziness improves — then have water and a snack.

The NHS adds the prevention version: get up slowly after sitting or lying down, and if you feel faint while standing still, "find a seat quickly and the faintness should pass — if it does not, lie down on your side".

The warning signs that you are about to faint

The HSE lists them, and knowing them buys you the seconds you need: feeling light-headed, feeling confused, blurred vision or spots in front of your eyes, ringing in your ears, sweating or feeling cold and clammy, feeling sick, and fast or unusually deep breathing.

If you do faint

The HSE describes what to expect: "If you faint you will usually be unconscious for 20 seconds. It is normal to feel disorientated for a few seconds after you wake. You may feel tired and weak for 30 minutes or more after fainting." Its advice is to rest, not to drive, and to get someone else to drive you home if possible.

Contact your maternity unit after a faint if you fell on or hurt your tummy, if your baby's movements have changed since, if you have high blood pressure or pre-eclampsia, or if you have tummy pain. Contact your GP or the out-of-hours service if you hit your head, or if this is the first time you have ever fainted.

The NHS's general advice on fainting adds the everyday context: it is usually caused by a sudden drop in blood pressure or heart rate, most people recover within a couple of minutes, and repeated faints are a reason to see a GP rather than to carry on.

When faintness is not just low blood pressure

The HSE's emergency list for dizziness in pregnancy is the part to memorise. Contact your maternity hospital or emergency department immediately if you are dizzy or feel faint and have:

Bleeding from your vagina.
Pain in your tummy.
Severe dizziness that does not go away.
Shortness of breath or chest pain.
An irregular or very fast heartbeat.
A severe headache or severe blurred vision.
High blood pressure, or a diagnosis of pre-eclampsia.

The HSE's separate list of warning signs in pregnancy overlaps with this one, and the NHS's list of symptoms needing help adds sudden vision problems and sudden swelling to the same cluster. Faintness with bleeding or one-sided tummy pain in early pregnancy is the combination that can indicate an ectopic pregnancy, and it is an emergency rather than a call-in-the-morning symptom. Faintness with a severe headache or vision changes belongs to the pre-eclampsia cluster. Faintness with breathlessness or chest pain belongs to the clot list.

When to call someone

Your GP or midwife if you feel dizzy or faint and have a mild headache or occasional slightly blurry vision (HSE).
Your maternity hospital or emergency department immediately for any of the combinations listed above.
Your local emergency number — 999 in the UK and Ireland, 911 in the US and Canada, 000 in Australia, 111 in New Zealand — if someone does not wake within a minute, has a fit or jerking movements, or is seriously injured in the fall.
Your midwife if you are feeling faint often. Low iron is common, treatable, and found with a blood test.

The short version

Falling blood pressure is a normal feature of pregnancy, and a low reading in a well pregnancy is not treated. Get up slowly, sit or lie on your side the moment you feel it coming, eat regularly, and stay off your back after 28 weeks. What changes the picture is company: bleeding, tummy pain, chest pain, breathlessness, a severe headache or vision changes all move this from ordinary to urgent.

Feeling Faint and Low Blood Pressure in Pregnancy