Before we discuss this topic it is important to know that any and all information found in this post is not medical advice and before you consider any type of activity, including diving or snorkeling, you should consult your physician.

Pregnancy can be a wonderful and exciting thing, but is does limit the activities you do to ensure the health of yourself and your developing baby. Most activities have a large body of research to draw from on the effects of continuing to do it while pregnant, but SCUBA diving is not one of those activities. The majority of research on this topic is done following the delivery of the child and from self-reported questionnaires on activities done during pregnancy. 
The current research suggests that the pressure put on the body during SCUBA diving in conjunction with the strain SCUBA diving may put on your body, especially if you are not an avid diver, and the risk of decompression sickness, make it unsafe for you and your baby. Studies have reported higher incidents of low birth weight, birth defects, infant respiratory problems, and other medical conditions when SCUBA diving was performed during pregnancy.
Freediving is a form of diving where the person holds their breath and dives down under the water and returns to the surface for air. The data on this sport is even more limited and comes mostly from freediving Asian cultures such as the Ama divers in Japan and Korea. These women live in a culture where they freedive for a living and they often do so until they deliver with no problems reported, but they do make changes as their pregnancies progress and some women stop diving completely. The potential for decompression sickness is not as big of a factor for freediving, but the stress on the body and the effects of extra pressure on the body definitely remain. Most doctors will recommend against any type of diving when pregnant, including freediving to limit the risk to you and your baby. Depending on your health, need to freedive, and the health of your baby, you may be able to continue to participate in this activity with changes to the depth and length of time your perform the activity and frequent monitoring of your pregnancy.
You may be able to snorkel while you are pregnant provided you remain on the surface and have ensured that everything is progressing normally in the pregnancy. Some places will not allow you to snorkel if you are pregnant and other places will, so be sure to check with your specific destination before booking a vacation if you are pregnant or trying to conceive. There is no research indicating that snorkeling on the surface will cause harm to the mother or the developing child but this is another activity that should be discussed with your doctor prior to being performed.
In the end, only you know your body and can decide what activities you simply cannot give up until your bundle of joy arrives, and what will be worth the wait. DAN recommends consulting with your doctor. DAN also has medics available to discuss this topic further. 

There comes a point in every SCUBA diver's life where they find themselves asking this very question. There are many pros and cons to both renting and buying and ultimately the decision usually comes down to your finances and your specific SCUBA activities. Here are a few of the pros and cons to both renting and buying so you can begin to answer this question for yourself!