What causes nausea in the third trimester of pregnancy?

Third Trimester Nausea: Causes and Mechanisms

Third trimester nausea occurs primarily due to mechanical compression of the stomach and intestines by the enlarging gravid uterus, combined with progesterone-induced slowing of gastrointestinal motility that persists throughout pregnancy. 1

Primary Physiological Mechanisms

Mechanical Factors

• The gravid uterus compresses the rectum and abdominal organs during the third trimester, creating physical pressure on the gastrointestinal tract that can trigger nausea 1
• This mechanical compression affects approximately 80% of pregnant women in the third trimester, similar to the prevalence of hemorrhoids from rectal compression 1

Hormonal Influences

• Progesterone levels continue to increase throughout the third trimester and reach a plateau, maintaining the slowed gastrointestinal motility that began in early pregnancy 1
• The persistently elevated progesterone causes delayed gastric emptying, meaning food sits in the stomach longer and can trigger nausea 2
• Rising estrogen and placental prolactogenic hormones in the third trimester are associated with more frequent symptoms, contrasting with the relatively calm second trimester when hormone levels stabilize 1

Fetal Contribution

• Carrying a fetus with certain genetic conditions may increase maternal symptoms through shared circulation via placental or fetal circulation, potentially affecting the mother's symptom burden 1

Clinical Pattern Differences from First Trimester

• Third trimester nausea differs from first trimester nausea and vomiting of pregnancy (NVP), which is driven primarily by elevated human chorionic gonadotropin and estrogen levels without the mechanical compression component 2
• First trimester symptoms typically resolve by week 16-20 in 80% of women, so nausea persisting or recurring in the third trimester suggests different underlying mechanisms 3
• Abdominal symptoms occur more frequently during pregnancy overall compared to the pre-pregnant state, making it important to distinguish physiologic third trimester nausea from pathologic causes 1

Important Clinical Caveats

• Always rule out serious pathology including preeclampsia, HELLP syndrome, acute fatty liver of pregnancy, gallbladder disease, or gastrointestinal obstruction when evaluating third trimester nausea 3
• The combination of mechanical compression and hormonal effects means that dietary modifications remain helpful: small, frequent meals prevent gastric overdistension, while high-protein, low-fat foods minimize delayed gastric emptying 2
• Unlike first trimester NVP, third trimester nausea is less likely to respond to antiemetics targeting hormonal pathways, since the primary driver is mechanical rather than hormonal 1, 2