Hormonal – related to the increasing amounts of estrogen and progesterone and their effect on the LES
Mechanical – the enlarging uterus increasing intra-abdominal pressure, inducing reflux of gastric acid
Behavioral – as with other instances of heartburn, behavioral modifications can exacerbate or alleviate symptoms

Unknown origin
Functional heartburn is heartburn of unknown cause. It is commonly associated with psychiatric conditions like depression, anxiety, and panic attacks. It is also seen with other functional gastrointestinal disorders like irritable bowel syndrome and is the primary cause of lack of improvement post treatment with proton pump inhibitors (PPIs). Despite this, PPIs are still the primary treatment with response rates in about 50% of people. The diagnosis is one of elimination, based upon the Rome III criteria. It was found to be present in 22.3% of Canadians in one survey. Diagnostic approach
Heartburn can be caused by several conditions and a preliminary diagnosis of GERD is based on additional signs and symptoms. The chest pain caused by GERD has a distinct burning sensation, occurs after eating or at night, and worsens when a person lies down or bends over. It also is common in pregnant women, and may be triggered by consuming food in large quantities, or specific foods containing certain spices, high fat content, or high acid content. In young persons (typically <40 years) who present with heartburn symptoms consistent with GERD (onset after eating, when laying down, when pregnant), a physician may begin a course of PPIs to assess clinical improvement before additional testing is undergone.
Resolution or improvement of symptoms on this course may result in a diagnosis of GERD.Other tests or symptoms suggesting acid reflux is causing heartburn include:

Onset of symptoms after eating or drinking, at night, and/or with pregnancy, and improvement with PPIs
Endoscopy looking for erosive changes of the esophagus consistent with prolonged acid exposure (e.g. - Barretts esophagus)
Upper GI series looking for the presence of acid reflux

GI cocktail
Relief of symptoms 5 to 10 minutes after the administration of viscous lidocaine and an antacid increases the suspicion that the pain is esophageal in origin. This however does not rule out a potential cardiac cause as 10% of cases of discomfort due to cardiac causes are improved with antacids. Biochemical
Esophageal pH monitoring: a probe can be placed via the nose into the esophagus to record the level of acidity in the lower esophagus. Because some degree of variation in acidity is normal, and small reflux events are relatively common, esophageal pH monitoring can be used to document reflux in real-time. Patients are able to record symptom onset to correlate lower esophageal pH with time of symptom onset. Mechanical
Manometry: in this test, a pressure sensor (manometer) is passed via the mouth into the esophagus and measures the pressure of the LES directly.Endoscopy: the esophageal mucosa can be visualized directly by passing a thin, lighted tube with a tiny camera known as an endoscope attached through the mouth to examine the oesophagus and stomach. In this way, evidence of esophageal inflammation can be detected, and biopsies taken if necessary.