Call now if this is you

The NHS puts sudden swelling on its list of pregnancy symptoms that need help, and names both reasons in one sentence: "A sudden increase in swelling can be a sign of pre-eclampsia or a type of blood clot called deep vein thrombosis (DVT) and needs to be checked immediately."

Contact your midwife, GP or labour ward straight away, at any hour, if you have:

A sudden increase in swelling in your face, hands or feet.
Swelling in one leg only, particularly with pain, warmth or a change in skin colour.
Swelling with a very bad headache, blurred vision or flashing lights, severe pain just below the ribs, heartburn that does not go away with antacids, feeling very unwell, or being sick.

Call your local emergency number — 999 in the UK and Ireland, 911 in the US and Canada, 000 in Australia, 111 in New Zealand — if swelling comes with chest pain, sudden breathlessness, pain on breathing in, or coughing up blood. The NHS adds: do not drive yourself to A&E.

Three questions that sort it out

Almost all of the triage comes down to three things, and you can answer all of them yourself before you phone.

1. How fast did it happen?

The NHS draws the line here: "Swelling that comes on gradually is not usually harmful to you or your baby, but it can be uncomfortable. A sudden increase in swelling can be a sign of pre-eclampsia."

Ordinary pregnancy swelling builds over weeks. It is worse at the end of the day and further into pregnancy, and better after a night's sleep. Swelling that was not there yesterday and is obvious today is a different event.

2. Where is it?

Ordinary swelling collects in the lowest parts of the body. The NHS: "Throughout the day the extra water tends to gather in the lowest parts of the body, especially if the weather is hot or you have been standing a lot." Feet, ankles and fingers.

Face and hands are the pattern that matters, and they are the ones the NHS lists first for an urgent call. Puffy eyelids in the morning, rings that will not come off, a face that looks different in a photograph — these are the ones people notice and then talk themselves out of.

3. One leg or both?

This is the question that separates the two serious causes, and it is the one most pages skip.

Pre-eclampsia swelling is generally symmetrical and involves the face and hands. A clot is not symmetrical. The NHS lists DVT symptoms as "throbbing pain in 1 leg (rarely both legs), usually in the calf or thigh", "swelling in 1 leg (rarely both legs)", "red, blue or darkened skin around the painful area — this may be harder to see on brown or black skin", and "swollen veins".

The HSE's version adds sensations you can test: "a heavy ache in the affected area" and "pain in your calf that gets worse when you bend your foot up towards your knee". It states plainly: "They usually only happen in 1 leg." It also flags a rarer presentation — a clot in the pelvic veins, where the NHS says "sometimes the only symptom you may have is pain in your lower tummy".

So: compare your legs. If one calf is visibly bigger, warmer or a different colour from the other, that is a same-day call regardless of how much swelling is present.

Why clots matter more in pregnancy

The HSE explains that pregnancy raises the risk because of changes to hormones, changes to circulation, and "pressure from your growing baby on the veins in the lower part of your tummy and pelvis" — and that "the risk continues for 6 weeks after you give birth". The NHS lists being pregnant, or having had a baby in the previous six weeks, among the situations that raise the chance of a DVT.

The HSE lists further things that raise it: a previous clot or a close family member with one, being over 35, a BMI over 30, smoking, severe varicose veins, poor mobility, twins or more, fertility treatment, dehydration or severe pregnancy sickness, a hospital stay, pre-eclampsia, a caesarean birth, heavy bleeding after birth, and travel over four hours without moving around.

ACOG's patient information covers prevention, and the practical points travel well: move around and flex your legs on long journeys, stay hydrated, and use compression stockings if they have been recommended to you. If you have risk factors, your GP or obstetrician may suggest treatment during pregnancy to prevent a clot — that conversation is worth asking for rather than waiting to be offered.

The pre-eclampsia side

Sudden swelling rarely travels alone in pre-eclampsia. The NHS's urgent list pairs it with a very bad headache, vision problems such as blurring or flashing lights, severe pain just below the ribs, heartburn unrelieved by antacids, feeling very unwell, and nausea and vomiting.

Two points worth holding on to. First, pre-eclampsia usually starts after 20 weeks or just after the baby is born — swelling in the days after birth still counts. Second, in the early stages there are often no symptoms at all, which is why blood pressure and urine are checked at every appointment. Symptoms appearing before anyone has recorded a high reading does not make them less significant.

What happens when you report it

For a suspected clot, the NHS says: "If a doctor thinks you have DVT, you should be referred to hospital within 24 hours for an ultrasound scan. The scan shows whether blood is flowing normally through the vein." RCOG's patient information covers the same pathway and the treatment, which is blood-thinning injections rather than the tablets used outside pregnancy.

For suspected pre-eclampsia, expect blood pressure, a urine test for protein, blood tests and monitoring of your baby, and possibly admission.

Both of those are ordinary responses to a phone call. Neither requires you to be certain before you make it.

When to call someone

Your midwife, GP or labour ward immediately for a sudden increase in swelling in your face, hands or feet (NHS).
Your midwife, GP or maternity unit urgently for pain, swelling, tenderness, warmth or redness in one leg, or a heavy ache in one calf (HSE).
Your maternity unit immediately for swelling with headache, vision changes, rib pain, unrelieved heartburn, nausea or feeling very unwell.
Your local emergency number for swelling with chest pain, sudden breathlessness, pain on breathing in, or coughing up blood. You can have a clot on the lung without any leg symptoms at all (HSE).
Your midwife in the six weeks after birth for any of the above. The risk does not end at delivery.

The short version

Gradual, symmetrical, evening-worse swelling of both feet is ordinary. Sudden swelling is not, and the NHS names two reasons for it. Face and hands, especially with a headache or vision changes, points at pre-eclampsia. One leg, especially with pain, warmth or colour change, points at a clot. Both get a phone call the same day.

Sudden Swelling in Pregnancy