Although the general rule is to avoid all kinds of chemical exposure during pregnancy, insect repellent is a notable exception. N,N-diethyl-meta-toluamide, commonly known as DEET, can protect you from mosquitos and ticks. Other insect repellents may also be effective, but DEET is the most effective repellent that is safe enough for use during pregnancy. [1,2]
The Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists (ACOG) are strongly recommending that all pregnant women use DEET to protect themselves and their developing babies from Zika virus.  There are many insect borne diseases you want to avoid during pregnancy, but Zika virus is by far the most dangerous. It can cause serious birth defects in about 30 percent of pregnant women who become infected. 
Zika and Other Insect Borne Diseases
Mosquitoes breed in warm, damp, and dark areas. Two types of mosquitoes cause most of the diseases you want to avoid. The female Anopheles mosquito transmits malaria. The female Aedes Aegyptus mosquito transmits zika, yellow fever, chikungunya, and dengue. These viral infections are common in many tropical areas of the world. However, as climate change had made the weather warmer and wetter, these diseases are spreading closer to home. 
Mosquitoes may be more attracted to pregnant women because pregnant woman give off more body heat, and mosquitoes are heart-seeking predators.  Zika was once only found in parts of Africa. It has now spread to Central and South America, Mexico, the Caribbean, and parts of Florida and Texas. [3,4] Although Zika is the biggest concern, other mosquito borne diseases can cause vomiting, diarrhea, and bleeding that can be dangerous during pregnancy. 
Lyme disease is a bacterial disease transmitted by ticks that live in wooded or high grass areas. Lyme disease is most common in the Northeast and Midwest. How Lyme disease affects pregnancy is not certain, but untreated Lyme disease during pregnancy may be linked to stillbirth, congenital heart and urinary tract defects, and rash or jaundice in newborns. 
How Safe Is DEET?
DEET has been used for many years in sprays, lotions, and oils. DEET is the only insect repellent that has been studied during pregnancy in humans. Most animal and human studies suggest that risk from DEET is very low when used as directed. According to the U.S. Environmental Protection Agency, DEET does not pose significant health concerns. In view of the known risks from diseases prevented by DEET, the CDC, ACOG, and many other health organizations strongly recommend using it if you need it. [1,2]
CDC also recommends the use of clothing treated with the insect repellant permethrin.  Another common mosquito repellent is picaridin. This repellent is also effective, but it has not been studied in human pregnancy. Animal studies suggest that it is also safe to use as directed. Picaridin may be an option if DEET is unavailable. You may also hear about natural mosquito repellents such as citronella, lemongrass, geraniol, or rosemary oils. These products may help keep mosquitoes away but they are not nearly as effective as DEET or picaridin. You should not rely on these products to protect you and your baby. [2,4]
How to Protect Yourself and Your Baby
The first thing to do is avoid areas where ticks and mosquitoes live. If you live in an area where you are protected from ticks and mosquitoes by winter, check with your health care provider before travelling to areas where insect-borne diseases are common. You can also check this CDC website for travel advisories: https://wwwnc.cdc.gov/travel.
If you are at risk and you need DEET, here is what you should do: 
- Look for a DEET insect repellent and read the directions carefully. A 25 percent solution provides about 10 hours of protection.
- Apply DEET only to exposed areas of your skin. Avoid any areas where there is a cut or irritated skin.
- Do not apply DEET under clothing. You can spray it on the outside of your clothing.
- If you are using a sunscreen, apply the sunscreen first and then apply DEET. Products that combine insect repellant and sunscreen are not as effective.
- Spray DEET into your hands and then rub it on your face. Avoid your eyes and mouth.
- When you are out of risk areas – back inside, in a screened in area – wash off the DEET with soap and water.
- The most common side effect from DEET is skin irritation. If this occurs, stop using DEET and talk to your healthcare provider.
What Else Can You Do?
Unfortunately, there are no vaccines that are safe during pregnancy to prevent these diseases. Insect repellents and avoidance are you best protection. In addition to using DEET, other protective tactics include: [2,4]
- Avoid being outside when mosquitoes like to bite, which is dawn to dusk.
- Wear clothing that covers as much of your body as you can stand. Remember you can spray DEET on your clothing or you can buy insect replant (permethrin-treated) clothing.
- Clean up any stagnant water around your house. This is where mosquitos breed and lay eggs.
- Make sure your living areas have air conditioning or screens. If not, sleep under a mosquito net.
- When walking in areas where ticks are active, stay on the paths, wear long pants, tuck your pants into your socks, and check for ticks before you come back inside.
Remember that the risk from mosquitoes and ticks, especially from misquotes carrying Zika, is much higher than any risk from using DEET. Use it if you need it. You can also use DEET when you are breastfeeding, just keep it way from your nipples and wash your hands after you apply. If you are pregnant in an insect-safe location, best not to travel into any areas where you might be at risk. Talk to your health care provider before you go. [2,4]
- Obstetrics & Gynecology, Insect Repellants During Pregnancy in the Era of the Zika Virus.
- MotherToBaby (Organization of Teratology Information Specialists), Insect Repellents.
- Obstetrics & Gynecology International Journal, Viral Mosquito-Borne Illnesses in Pregnancy: A Look at Aedes.
- What To Expect, How to Safely Protect Yourself from Mosquito Bites During Pregnancy.
- March of Dimes, Lyme Disease and Pregnancy.