The final weeks of pregnancy often lead expectant mothers to scrutinize every physical change. Nausea, a feeling of stomach upset usually associated with first-trimester morning sickness, frequently resurfaces late in the third trimester. This symptom is a recognized, though not universal, physical manifestation of the body preparing for childbirth.
Nausea: A Common Sign of Impending Labor
Nausea can serve as a preliminary indication that the body is initiating labor, often appearing as part of non-specific symptoms. This pre-labor nausea typically manifests shortly before true contractions begin, usually within 24 to 48 hours of active labor.
This symptom rarely occurs in isolation; it frequently accompanies other physical signals of impending change. Many women also report loose stools or diarrhea, sometimes described informally as the body “clearing house.” Other common signs include the “nesting instinct,” or the feeling of the baby dropping lower into the pelvis. While these symptoms do not confirm that labor is imminent, their presence alongside nausea suggests that the birthing process is drawing near.
The Physiological Causes of Pre-Labor Nausea
The biological mechanism driving late-pregnancy nausea is fundamentally different from the hormonal surge causing morning sickness in the first trimester. As the body transitions toward labor, a significant shift occurs in the balance of hormones maintaining the pregnancy. Specifically, the functional decline of progesterone, the hormone responsible for keeping the uterus relaxed, allows the uterine muscle to become more irritable.
As progesterone levels shift, the body increases its production of prostaglandins. These lipid compounds act locally to ripen the cervix and stimulate uterine contractions. Prostaglandins are not confined to the uterus; they affect smooth muscle tissue throughout the body, including the gastrointestinal tract. This systemic action irritates the digestive system, resulting in the nausea and diarrhea commonly experienced just before labor begins.
Another contributing factor is the slowing of gastrointestinal motility. As the body conserves resources for labor, the digestive process becomes less efficient. This slowdown, combined with the powerful muscular contractions of the uterus, can place mechanical pressure on the stomach and intestines, contributing to feelings of queasiness or vomiting as labor progresses.
Distinguishing Labor Nausea from Other Late-Pregnancy Concerns
While nausea can be a normal part of the pre-labor experience, it is sometimes difficult to differentiate it from other benign or serious late-pregnancy conditions. Late-term pregnancy frequently involves issues like severe heartburn or gastroesophageal reflux, where the enlarged uterus pushes stomach acid upward. Simple dehydration or exhaustion in the final weeks can also easily trigger feelings of nausea.
It is important to recognize when nausea is accompanied by specific symptoms that could indicate a more serious medical complication. Severe nausea and vomiting should be immediately reported to a healthcare provider if accompanied by a persistent, intense headache that standard pain relief does not improve. Other concerning signs include sudden swelling of the face or hands, visual disturbances, and pain in the upper right side of the abdomen. These combined symptoms may be indicative of preeclampsia or its severe variant, HELLP syndrome, conditions that require urgent medical evaluation.