Editor's note: Emily Oster is the author of "EXPECTING BETTER: Why the Conventional Pregnancy Wisdom is Wrong -- and What You Really Need to Know." Oster is an assistant professor of economics at the University of Chicago. She received her Ph.D. in economics from Harvard University.
(CNN) -- When I got pregnant with my daughter three years ago, I found myself drowning in frequently alarmist pregnancy advice.
I definitely got the impression there was a lot to worry about, but no one seemed to be able to agree on exactly what those worries should include.
The books didn't always agree with my doctor, my doctor didn't always agree with the official recommendations, and the Internet never agreed with anyone. I quickly realized that medical recommendations come from studies, and I could use my training -- in economics and statistics -- to evaluate those studies and get to the bottom of the pregnancy rules.
Sometimes what I found was exactly what I expected, and sometimes it was a little different. Here's a taste of what I found:
Myth 1: Caffeine is off limits
Here is where I really found the recommendations differed. Many books (and my OB) said a moderate amount was fine, but others said no caffeine at all.
The big risk with caffeine, we are told, especially early in pregnancy, is miscarriage. When I got to the actual studies, however, I found that there is no evidence of that risk from consuming less than 200 milligrams of caffeine per day -- about two cups of coffee. I also found that most of the evidence supported even up to three or four cups a day.
There did seem to be some evidence of higher miscarriage rates at six to eight cups a day, but I decided I could live with my three-cup-a-day habit.
Myth 2: The type of prenatal testing you do should differ if you are over or under 35
Prenatal testing is a complicated decision for many women, forcing us to confront concerns about a disabled child and risks of miscarriage.
A common recommendation is that women over 35 should undergo more extensive testing, typically an amniocentesis or CVS test to look for genetic abnormalities (both tests carry some risk of miscarriage). Women under 35 are usually told to undergo a screening procedure that carries no risk but provides less information.
This cutoff age is based on the fact that the risk of chromosomal problems increases with the age of the mother. At 35, the probability of a chromosomal problem is about equal to the risk of miscarriage from the amniocentesis.
In fact, this decision should be more about the preferences and values of the pregnant woman, and her spouse, than about an age cutoff. Women need to consider their personal assessment of the risks of a miscarriage versus a developmentally delayed child when making this choice. The American College of Obstetricians and Gynecologists agrees.
Myth 3: Having sex can jump-start labor
Although there is a chemical reason to think this might work, randomized trial data doesn't support it. When doctors randomly encouraged couples to have sex late in pregnancy, women didn't go into labor any faster than couples who weren't encouraged to do so.
On the other hand, nipple stimulation has been shown to bring on labor, but it takes a few hours a day.
Myth 4: Having an epidural lengthens your labor
TRUE (but small)
Talking to women about birth can be polarizing. I heard claims that having an epidural increases labor time by hours and, on the other hand, claims that there is no such effect.
The truth was in the middle: Having an epidural does lengthen the pushing stage of labor, on average, by about 15 minutes, according to a 2011 study. Of course, you may not care as much given the pain relief.
Myth 5: Unpasteurized soft cheese are off limits
The biggest food-related risk in pregnancy is listeria. It's a dangerous bacteria, to which pregnant women are especially susceptible, that can lead to miscarriage or stillbirth.
Concern about listeria contamination is the reason that unpasteurized soft cheeses are off the menu for pregnant women. When I looked into it, I found that this is probably a sensible restriction. About 20% of listeria cases in the United States over the past 15 years were caused by raw milk soft cheese (usually queso fresco, a Mexican soft cheese).
It is worth noting, however, that this is all about unpasteurized cheese. So feel free to enjoy some Brie -- just make sure the milk it is pasteurized.
The opinion expressed in this article are solely those of Emily Oster.