Although perfectly safe to eat during pregnancy, Ketchup (also known as catsup in some countries) can aggravate gastrointestinal symptoms such as heartburn or reflux due to the high level of acidity. Commercial ketchup often has high amounts of sugar. Some women tend to have more aggravated symptoms of heartburn or acidity during pregnancy, therefore it is better to consume in moderation.
Watch out for the dried up ketchup collected around bottle cap & mouth that tends to harbour bacteria and may contaminate the ketchup itself.
Since the key ingredient in ketchup is tomatoes, also read about having tomatoes during pregnancy here: