Hi,
Thanks for the query.
The possible causes are:
High blood pressure as in pre-eclampsia especially if associated with HELLP syndrome etc.
liver disease leading to alteration in coagulative factors, formation of dilated blood vessels at the esophageal opening,
In case of severe vomitings esophageal mucosa can get abrasions which can cause blood in vomitings but this will be little in amount usually.
So better to consult gynecologist as early as possible to rule out high blood pressure etc, as sometimes these can lead to more serious conditions like eclampsia.
Take care.

hello there..
Vomitting out blood means she has hematemesis.
Most common it can be due to severe gastritis and peptic ulcers. Also liver failure in pregnnacy which occurs in HELLP syndrome can lead to this.It can be due to many other reasons. Depending on the site, symptoms vary like:
Upper GI bleeding (from esophagus, stomach, duodenum) may also result in melena (dark blood in the stool) or if the bleeding is profuse, even hematochezia (fresh blood in the stool) may be noted.
Mild GI bleeding that is prolonged or excessive bleeding may result in anemia, hypotension (low blood pressure), dizziness and fainting.
Abdominal pain may also be noticed especially if the bleeding is due to peptic ulcers, erosive gastritis, Mallory-Weiss tear(due to excess vomitting) or chemical poisoning.
Hematemesis accompanied by the collection of fresh blood in the mouth may be indicative of a rupture of esophageal varices.
Vomiting non-bloody vomitus or retching may be followed by a bloody vomit which would indicate a Mallory-Weiss tear or rupture of esophageal varices as a result of the preceding forceful vomiting or retching
Esophagitis
Esophageal cancer,rupture/tear
Esophageal varices
Mallory-Weiss tear
Gastritis
Stomach cancer
Gastric ulcers (Peptic ulcers)
Stomach varices
Mallory-Weiss tear
Duodenitis
Duodenal ulcer
Arteriovenous malformation
Benign tumors of the esophagus, stomach or duodenum
Alcohol abuse
Cirrhosis( chronic liver disease)
Drugs like NSAIDs (non-steroidal anti-inflammatory drugs)
Liver failure
HELLP syndrome in pregnancy (which is a complication of raised BP in pregnancy)
Pancreatitis
Pancreatic cancer
Poisoning with ingested toxins like in arsenic poisoning
Portal hypertension
Bleeding disorders rarely.

I suggest she should visit a physician at the earliest . Get certain blood test including Coagulation profile with liver function tests done. Also an ultrasound abdomen with antenatal ultrasound is a must to know the site of bleeding along with the present health of the baby.

She should not delay treatment.