Holly McMillan is 40 weeks pregnant, expecting her baby girl to be born any minute. Her daily walks have been tougher since summer began.

"Yeah, you definitely sweat a little more than normal. The extra weight that you're carrying, you tend to be a little more aware of when it's 90° outside than when it's 70° outside," says McMillan.

She can go a few miles if it's early in the day, but she listens to her body and knows when to call it quits.

"If I'm walking I can usually tell when I'm getting a bit more winded and I'll track my heart rate on my fitness tracker," adds McMillan.

She also uses an app to keep track of her water intake.

McMillan's doctor, Kirk Brody says hyperthermia, especially during early pregnancy, can increase the risk of birth defects in the brain or spinal cord.

"They're hot in the winter much less in the winter much less when it's 90° or 100° outside," explains Brody. "So, it's really important that they find some shade if they can. I try to tell my pregnant ladies it there's things you have to do outside, try to do them in the early morning or late evening."

Overheating can cause dizziness and falls which could harm the baby.

Brody says to keep your body temperature at less than 103°, and drink plenty of water throughout the day.    

"If you're not having to urinate at least every couple hours as a pregnant lady, you're not drinking enough. If the urine's getting really strong or really yellow, again you're not drinking enough," says Brody.

Dehydration also makes the brain produce a hormone called vasopressin, which is similar to a hormone involved in stimulating contractions.

"You'll get these annoying contractions that will force you to call your doctor, or go to the office, or go to the hospital only to find out you're not really in labor."

Brody also says that pregnant women should not use saunas. Hot tubs are safe if the water temperature doesn't get warmer than 100°, and limit your dip to 10 minutes. Lastly, apply and reapply sunscreen often while outdoors.