Economist, mom, and author Emily Oster, PhD answers your question. Follow us on Instagram to ask your questions!



By Author Emily Oster, PhD

Emily Oster is a Professor of Economics at Brown University. She holds a PhD in Economics from Harvard. Prior to being at Brown she was on the faculty at the University of Chicago Booth School. Oster’s academic work focuses on health economics and statistical methods. This month, she answered all your questions. 

Drinking while TTC around ovulation?

Keep it in moderation. Evidence suggests if you drink a lot in this period it can affect the ability to get pregnant. 

What’s the deal/data on prenatal vitamins?

By far the most important element of prenatal vitamins is the folic acid. Being deficient in folic acid can lead to birth defects, so it’s important to keep your levels up before pregnancy and during early pregnancy.  For some women, anemia is also an issue in pregnancy so iron pills can help.  The evidence on the rest of the elements of these supplements is pretty weak (but they won’t hurt you!

Does marijuana affect your fertility? 

We don’t really know.  It’s really hard to study impacts of marijuana in pregnancy because (until recently) it has been illegal in most places. This means that there is a stigma to reporting usage, so there are a lot of differences in women who report using marijuana and those who do not.  This makes it very hard to estimate the impacts on conception (or on anything else).  As more places legalize, we may learn more.

What’s the real scoop on drinking wine in early pregnancy? 

The best data doesn’t point to risks of occasional drinking in early pregnancy.  This means occasional - one to two drinks a week.  There’s a lot more to reading these data and coming to this conclusion, but that’s where it ends up. 

Can I drink coffee during pregnancy?

Yes!  The concern about coffee during pregnancy really stems from a concern about higher rate of miscarriage. But when we look at the data, it doesn’t support that concern at low levels (I’m talking up to 3 or 4 cups a day).  For women who drink a lot of coffee (like, more than 5 or 6 cups per day) the evidence on miscarriage is more mixed. This argues for moderation, but definitely not elimination.

Is weight gain different based on your BMI?

Generally, yes. Roughly, if you weigh more before pregnancy the recommendation is to gain less weight. So it’s 25 to 35 pounds if you have a normal BMI, and less if you are classified as overweight or obese.  One way to think about this logic is if you are in the overweight or obese category they would recommned you lose weight if not pregant, so some of that is reflected in the recommendations. 

What acne products can I use during pregnancy?

I’m revising Expecting Better to include skin care stuff now!  Short answer is that some drugs like accutane are really off limits, whereas other treatments (benzoyl peroxide, antibiotics) are considered safe.  Topical retinoids -- like Retin-A -- are somewhere in the middle.  Most data is reassuring, but there is enough concerning evidence that it’s not completely clear and generally is not recommended. 

📖 Shop Expecting Better ⟶

My OB wants to induce at 41 weeks. Is that better for first time low risk mom? 

Yes, there’s new data out from Sweden which shows that it is safer to induce at 41 weeks than to wait until 42 weeks.  Stillbirth appears to be more common for women who wait longer. It’s very rare in any case, but for most women it seems likely that it’s better to avoid going past 41 weeks.

How can I tell if I have PCOS/ if I'm going to struggle to conceive?

Great question! Read Natalist's Guide to PCOS to understand more.

How safe is it to smoke just a bit of weed while breastfeeding (after the baby is sleeping)?

I wish I could tell you for sure!  We don’t really know. Use of marijauana while breastfeeding has been linked to lower developmental outcomes in kids, but it’s really hard to know whether this is the weed or other differences across moms. We’re going to need to wait for better data, which we will hopefully have as more areas legalize. 

Details about breastfeeding journey & what feedings looked like in the first three months till you got it?

How long do you have?  Seriously, I had such a hard time with this in the first months, especially with my daughter. It took forever for my milk to come in, and then I was pumping all the time and also trying to feed her. She never seemed satisfied, and would only latch if she was basically asleep or if I sang and bounced her while trying to get her to latch. It was exhausting and demoralizing.  She did finally “get it” (or, rather, we did) at around 3 or 4 months. But it never felt that natural to me.  My second was a dream relative to this...

What does current data say about babies and flying?

The main risks to babies in flying is exposure to germs. There are good reasons to try to avoid illness in the first months, because infants are more susceptible but also because the reaction to a fever is likely to be medically complicated (i.e. it may need a spinal tap).  So, if you can, wait until a couple of months (after the first shots) before flying. If you need to fly before that, try to keep the baby protected from germs. 

If you can, wait until a couple of months (after the first shots) before flying.

How long do you have to breastfeed to see the decreased cancer risk? 

To be clear: the cancer risk that is declining is breast cancer for mom.  The risk seems to scale with time nursing, so more is better. But worth noting that all of these effects are fairly small.

Fed is best, but are formula fed babies at a disadvantage? 

The best data shows that there are some short-term benefits to breastfeeding, including lower risk of rashes, better gastrointestinal health and possibly fewer ear infections.  These all show up in the first year. There isn’t good evidence to support many of the longer term benefits to health or IQ. The overall picture is much more balanced than much of the rhetoric would have you believe.

Room sharing with a baby - what are the facts? 

There is some fairly weak evidence suggesting that room sharing (but notably NOT BED SHARING) with a baby lowers the risk of SIDS.  I say “fairly weak” because this evidence mostly comes from “case-control” studies which have a hard time addressing concerns about recall bias and difference across families.  To the extent there are benefits, they seem to appear in the first few months.  And it’s worth noting that infant who are in their own room by 4 months do seem to sleep better at 9 months, and even at older ages.

Exclusively pumping vs breastfeeding - does how the baby gets the breast milk matter? 

In general, the benefits of breastfeeding are more limited than often claimed. But to the extent they are there, there is nothing in the data which would suggest that the method of delivery matters.  It’s not that it couldn’t, although it’s hard to really see how it would, but our data is just not sufficient to understand this. There is surprisingly little evidence on exclusive pumping!

In general, the benefits of breastfeeding are more limited than often claimed.

Is there any data out that indicates the ‘ideal’ parental leave policy? Do countries with longer time off have higher female workforce participation? 

This is a super interesting question that some of my economics colleagues could nerd out on forever!  The short answer is that it seems like very long parental leave may actually make things a bit worse, but too short or no leave is bad.  The benefits to kids seem to accrue from the first four months of leave.  But when there are very long leaves, it’s tough for employers and may actually impact initial hiring of women.  No easy answers...