Call your maternity unit immediately if

Feeling sick in the third trimester is usually mechanical. It is also on the pre-eclampsia list, and that possibility has to be excluded before anything else is considered.

The NHS urgent-advice box says to call your midwife, GP or labour ward immediately if you have any of:

feeling sick (nausea) and being sick (vomiting)
a sudden increase in swelling in your face, hands or feet
a very bad headache
problems with your vision, such as blurring or flashing lights in your eyes
severe pain just below your ribs
heartburn that does not go away when you take antacid medicine
feeling very unwell

It names the reason: "These could be symptoms of pre-eclampsia, which can lead to serious complications if it's not monitored and treated."

The HSE frames the same cluster for the third trimester: contact your GP, maternity unit or hospital if you have "pain higher up in your stomach or chest, or you have a headache, swelling of your hands, feet and face or blurring of your vision — these are signs of pre-eclampsia". RCOG's patient information lists vomiting among the symptoms that should prompt urgent contact.

Two other same-day patterns: the HSE says itching, severe nausea or vomiting, or the whites of the eyes turning yellow "could be signs of a liver or gallbladder problem"; and a fever of 38C or higher alongside sickness suggests infection.

Now the ordinary explanation

Once those are excluded, most late nausea is mechanical and there is nothing sinister about it.

By the third trimester the uterus occupies the space the stomach used to have. The consequences are predictable: you feel full after a few mouthfuls, food sits rather than moving on, reflux is easy, and nausea follows. People often describe feeling sick specifically after eating, after lying down, or in the evening — all of which point at the mechanics rather than at anything systemic.

Heartburn is the other major driver. Acid at the top of the oesophagus produces genuine nausea, and it is worst at the same times: after meals, and lying flat. Note, though, that the NHS specifically names "heartburn that does not go away when you take antacid medicine" on the pre-eclampsia list. Heartburn that responds normally is heartburn. Heartburn that will not respond is on a different list.

How it differs from morning sickness

The NHS says nausea and vomiting are very common in early pregnancy and that for most people it improves or stops by around weeks 12 to 20. Late nausea has a different shape:

It is provoked by eating rather than by an empty stomach.
It is worse lying down, whereas early sickness is often worst on waking.
It is usually accompanied by fullness and reflux rather than by smell sensitivity.
It fluctuates with the baby's position and often eases if the head engages.

The one that means labour

Nausea and vomiting are a recognised part of early and established labour, and vomiting is common in transition. The NHS lists the signs that labour has begun as contractions or tightenings, a show, backache, an urge to go to the toilet caused by the baby's head pressing on the bowel, and waters breaking.

Sickness arriving alongside regular tightenings, low backache or a show is worth reading as labour rather than as illness. Before 37 weeks, the NHS instruction is unambiguous: call your maternity unit immediately with regular contractions or tightenings, period-type pains, a gush or trickle of fluid, or backache that is not usual for you.

What helps

Smaller meals, more often. The single most effective change, because it addresses the actual mechanism.
Stay upright after eating. Sitting up straight, and not lying down for a while after a meal, keeps stomach contents where they belong.
Prop yourself up at night. Extra pillows under the head and shoulders reduce overnight reflux.
Avoid the usual triggers — high-fat and spicy foods, chocolate, citrus and caffeinated drinks are the ones national guidance names for heartburn.
Sip fluids between meals rather than with them, so that liquid is not competing for the space food needs.
Ask about antacids. Antacids and alginates are commonly used in pregnancy, but ask your midwife or pharmacist which is suitable rather than choosing off the shelf.

Other explanations worth ruling out

Two more causes account for a good share of late sickness. A urine infection can present with nausea and little else, which is one reason a urine sample is taken at every appointment and worth requesting if you feel unwell. And a stomach bug is a stomach bug at any gestation — the distinguishing feature is that it arrives suddenly, usually with diarrhoea, and passes within a couple of days rather than tracking your meals.

Gallbladder problems also become more common in pregnancy and produce nausea alongside pain in the upper right abdomen, often after fatty food. The HSE names this pattern for the third trimester, grouping itching, severe nausea or vomiting and yellowing of the whites of the eyes together as signs of a liver or gallbladder problem.

When sickness itself is the problem

If you are vomiting repeatedly and cannot keep fluids down, that is not something to manage at home regardless of the trimester. The NHS advises telling your midwife or doctor, or contacting the hospital as soon as possible, because of the risk of dehydration. It lists the signs as feeling thirsty, tired, dizzy or light-headed, not peeing very much, and dark yellow, strong-smelling pee.

Sickness that persists into or returns in the third trimester is also worth reporting simply so it is on record, because it is one of the things a midwife will want to weigh alongside your blood pressure and urine result at the next appointment.

Feeling Sick Again in the Third Trimester