Foot pain affects around 50% of pregnant women. A large number of those are plantar fasciitis.
The most obvious reason is the weight gain during pregnancy. As the pregnancy progresses the weigh that the feet have to carry increases and that increases the load on the feet in general and the plantar fascia in particular increasing the risk for plantar fasciitis. Plantar fasciitis is more common in those who are overweight, so there should be no surprise that the incidence is higher during the weight gain of pregnancy.
The increased weight is not the only reason for foot pain during pregnancy. The feet also swell during pregnancy due to the fluid retention and this can cause issue with the fitting of footwear. A hormone called relaxin is also released in increasing amounts during the pregnancy which is aimed to loosen ligament around the pelvis in preparation for child birth. That hormone also affects ligaments elsewhere in the body including the foot. That means that the increased wight during pregnancy is imposed on the foot that looses some of its ability to support that weight due to the ligaments relaxing.
What can be done to relieve foot pain during pregnancy? Exercise is important, especially before the pregnancy if you are planning on getting pregnant. Lots of strengthening and stretching for the muscles and the feet are a good idea, so it can take the increased weight. Wear supportive footwear ad even use some arch supports. Elevate the feet often to help with the swelling and use compression hosiery if needed. Lots of foot massages during the pregnancy will help the aches and pains that develop in the feet.
If you develop plantar fasciitis during pregnancy, you may need to see a podiatrist to get professional help at helping with the symptoms.