From the Guidelines

Lactulose and polyethylene glycol–based laxatives are safe for pregnant patients with constipation.

Dietary fiber intake is recommended to promote regular bowel movements and prevent constipation, with a daily amount of approximately 30 g/d of fruits, vegetables, whole grains, and legumes 1.
Bulk-forming agents, such as psyllium husk or methylcellulose, are safe to administer in pregnancy due to the lack of systemic absorption 1.
Osmotic laxatives, such as polyethylene glycol or lactulose, can also be administered safely during pregnancy, but may cause maternal bloating 1.

Stimulant laxatives should be avoided because safety data on these medications are conflicting 1.

Encouraging pregnant women to avoid straining during bowel movements by providing ample time, using relaxation techniques, and adjusting diet and hydration is helpful 1.
Consumption of an adequate amount of fluids, particularly water, can help soften stools and ease bowel movements 1.

From the Research

Natural fibres (bulk-forming agents), docusate sodium (stool-softener), mineral oils (lubricant laxatives), macrogol (polyethylene glycol, PEG), sugars and sugar alcohols (osmotic laxatives) and anthraquinones and diphenolic laxatives (stimulant laxatives) seem to be safe medicaments regarding teratogenicity and lactation 2
Macrogol and lactulose are suggested as the first-line therapy in treating chronic constipation during pregnancy, with macrogol showing some advantages such as faster onset of bowel action and fewer flatulences 2
Diphenolic laxatives such as bisacodyl and sodium picosulfate are recommended as a second-line treatment in the second and third trimester 2
Fibre supplements, such as bran or wheat fibre, are likely to help women experiencing constipation in pregnancy, and stimulant laxatives may be more effective than bulk-forming laxatives but may cause more side effects 3, 4
Osmotic laxatives, such as polyethylene glycol, may be beneficial for some patients, and the short-term use of osmotic or stimulant laxatives is generally reserved for patients who fail to respond to dietary changes or bulking agents 5

Stimulant laxatives versus bulk-forming laxatives: stimulant laxatives appear to be more effective in improvement of constipation, but are accompanied by an increase in diarrhoea and abdominal discomfort 3
Fibre supplementation versus no intervention: fibre supplementation may increase frequency of stools compared with no intervention 3

First-line therapy includes increasing fibre intake through diet or supplements 5
Alternative solutions, such as proposals of the American College of Gastroenterology's Committee on FDA related matters, (ACG-FDA) and the Motherisk Programme, may help improve decision-making for the right laxative 2