For most pregnant women, nausea and vomiting are most likely to develop in the first month or two of pregnancy and begin to subside at the end of the first trimester, at week 18 on the outside. But up to 20 percent of women may find that their nausea symptoms during pregnancy last until the third trimester, and 5 percent experience nausea and vomiting just before delivery.

For some women, morning sickness goes away after a few weeks early in their pregnancy, only to return again around week 27 or later. Still other women have no nausea at all in the early stages and are quite surprised that it develops in the last three months.

Of course, at any time, the sudden onset of nausea and vomiting can be due to a virus or another illness. Especially if it is accompanied by diarrhea, it could be the flu or food poisoning. If these symptoms are severe at any time during pregnancy, it is important to watch out for dehydration and make sure to replenish the fluids that are lost.

Starting at week 27, hormonal activity tends to pick up, having stabilized somewhat for most women during the middle stages of pregnancy. Women who maintain high levels of the hormone hCG are particularly likely to find that their NVP continues throughout pregnancy as well.

Another instigator of nausea and vomiting in the later stages of pregnancy is simply the size of the growing fetus. At this time, it is more common for nausea to arise immediately after a meal, often accompanied by heartburn. As the baby grows, the uterus puts more pressure on the stomach causing heartburn and nausea when the stomach is full. At this point, the stomach’s capacity is even less, and it’s even more important to stick to smaller meals to reduce this stress.

So nausea in the third trimester is not necessarily unusual or worrisome. However, there are a couple of serious liver diseases in late pregnancy that can also lead to nausea and vomiting, including acute fatty liver of pregnancy and HELLP syndrome. Although both ailments are really rare, they are also life threatening if left untreated. For this reason, doctors now recommend that any woman experiencing nausea and vomiting accompanied by upper abdominal pain and “discomfort” in the third trimester of pregnancy have her liver enzymes and kidney function evaluated, along with a complete blood count.

Also, any woman experiencing nausea and vomiting in combination with severe headaches, facial swelling, sudden bloating, abdominal pain, and visual disturbances (eg, seeing spots) should call her doctor immediately. These are signs of pre-eclampsia, a disease that can develop fairly quickly at any time after the 20th week of pregnancy and affects up to 8 percent of pregnancies. Preeclampsia can cause strokes and kidney damage, liver failure, fuzzy clots, fluid in the lungs, seizures, and even death for both the child and the mother.

There is a final factor that can cause nausea in late pregnancy, although in this case it is a little easier to diagnose. If nausea, vomiting, and diarrhea coincide with cramps, back pain, pelvic pressure, and contractions that occur less than 10 minutes apart, labor has begun.

While not uncommon, then, nausea at the end of any pregnancy warrants a consultation with your care provider. In most cases, there are ordinary and harmless reasons for it, but there is an external possibility that it is a symptom of one of these several serious illnesses that occasionally arise during the last weeks of pregnancy.