Senna is a natural herb that has been used for centuries to make tea. It is made from the Cassia plant that grows in the Middle East and India. The reason people have been drinking senna tea for centuries is because it is very effective at relieving or preventing constipation. 
Other than nausea, constipation is the most common complaint of pregnant women. Since close to 40 percent of pregnant women may experience constipation, drinking senna tea can seem like a safe and natural option.  But remember, natural does not always mean safe. Socrates died from drinking the natural herb hemlock.
Senna for Constipation
Senna tea will not kill you and neither will constipation, but they can both make you uncomfortable. Senna is used as an over-the-counter (OTC) medicine for constipation. In addition to tea, it can be taken in pill or powder form. [1-3]
It works through chemicals in the senna plant called sennosides that irritate the lining of your colon, causing water to rush in and stimulation of colon muscles to push stool out. No wonder side effects can include cramps and watery diarrhea. This type of laxative is called a stimulant laxative, and it is one of the last choices for pregnant women. [1-3]
Is Senna Tea Safe During Pregnancy?
You may have read that senna tea can increase the risk of premature contractions. There is little evidence to support this. There is no evidence that senna increases your risk for birth defects.  During pregnancy, senna is considered possibly safe for short term use. It is considered possibly unsafe for long-term use. [1,3]
Drinking senna tea for more than two weeks can cause diarrhea and dehydration. This can wash out important blood minerals like potassium, which is important for heart and muscle function. In very large doses, senna may cause liver damage. Occasional use of senna tea is safe during breast feeding. Although some senna can cross into breast milk, it is not enough to affect a baby’s digestive system. [1,3]
Better Choices for Constipation
Constipation occurs during pregnancy for many reasons including hormone changes, decreased activity, increased absorption of water from the intestines, and the constipating effects of minerals like iron and calcium that are included in prenatal vitamins. Later in pregnancy, the increased size of your womb can put pressure on your colon and add another constipation risk. 
First line therapy for constipation is to increase fibber, exercise, and fluids. If these measures do not help, the next step is laxatives. The safest choices are bulk forming laxatives (psyllium or bran), lubricants (mineral oil), and stool softeners (docusate sodium). If you are still constipated, you need to talk to your doctor before moving on to more powerful laxatives like an osmotic (magnesium sulfate, lactulose, or citrate) or a stimulant laxative (bisacodyl or senna). 
There are a few other warnings about senna tea or a senna OTC. Senna can interfere with birth control pills and decrease effectiveness of birth control. Because senna can decrease potassium levels it should not be used with the heart medication digoxin. It may also interfere with estrogen replacement therapy, water pills, and some blood thinners. [1,3]
Do not use senna along with the herb horsetail. Horsetail is used to reduce fluid retention, which can be a problem during pregnancy. Combining horsetail with senna can increase the risk of dehydration and potassium loss. In any case, horsetail has not been studied in pregnancy, so it can not be considered safe. Do not use senna with licorice. Some women may use licorice to relieve heartburn, another common complaint in pregnancy. Like senna, licorice can deplete potassium, so they are a bad combination. 
Bottom line on drinking senna tea during pregnancy is that there are much better choices. If your doctor says it is OK for you, do not use it for more than two weeks. Make sure to also drink an electrolyte replacement fluid like Gatorade to prevent dehydration and potassium depletion. 
Medical News Today, Is senna tea safe to drink?
Canadian Family Physician, Treating constipation during pregnancy.