by: Lily Nichols, RDN, CDE
I never intended to write about intermittent fasting and pregnancy. And yet, here I am.
In the past few months, I’ve been asked about intermittent fasting and pregnancy more times than I can count and it’s time to address this topic head on.
First off, let’s make sure we get our terms straight.
What is Intermittent Fasting?
Intermittent fasting (IF) is the choice to voluntarily abstain from eating for periods of time (fasting) while choosing to eat during other periods of time. The length of the “fasting window” or “eating window” varies, with some choosing to fast for 16+ hours per day, while others may go 24 hours or longer.
The most popular version at the moment (it’s 2019 at the time of writing, just FYI) is a 16:8 fast, meaning you eat all of your food for the day within an 8 hour window and abstain from food for the remaining 16 hours in the day (water and sometimes other non-caloric fluids are “permitted” during the remaining 16 hours per day).
This is often accomplished by skipping either breakfast or dinner. Intermittent fasting may be, but is not always, combined with caloric restriction. That means you might consume the same amount of food, just within a shorter period. Or, you may intentionally or unintentionally eat less than usual.
It is also often, but not necessarily, combined with specific diets. Many low carbers, for example, combine this way of eating with some variation of intermittent fasting (intentionally or unintentionally), since a lower carb diet is often more satiating, keeping hunger at bay.
Why do People Choose Intermittent Fasting?
There’s a fair amount of data (mostly from adult men) showing that fasting can improve body composition, blood sugar metabolism/insulin sensitivity, promote weight loss, improve cognitive function, clear out dead/damaged cells (aka stimulate “autophagy”), and reduce inflammation. This article has a comprehensive overview of the topic.
The above benefits are all well and good, however we always need to consider CONTEXT when thinking about whether or not a specific health practice is beneficial to us or not given our unique set of circumstances. One of those special circumstances is pregnancy.
Note that nearly all fasting experts I’ve talked with do not condone fasting during pregnancy.
When I spoke at Low Carb Denver on the science and controversy surrounding low carbohydrate diets and pregnancy, I was asked repeatedly by attendees about my opinion of intermittent fasting and pregnancy.
I usually responded with the following:
“Before I share my thoughts, I’d love to hear what makes you ask this question.”
“That’s an interesting question. Do you have any personal or professional experience so I can understand how this topic got on your radar?”
Their answer was often some version of a personal beneficial experience with fasting (outside of the context of pregnancy) or a professional interest in the research (or lack thereof) on fasting with regards to pregnancy (they’re usually well aware of the non-pregnancy research on fasting).
Then I’d usually follow up with some questions about their personal/professional experience specifically with pregnancy.
Intermittent Fasting and Pregnancy: Why it’s a Mismatch
If you’ve ever been pregnant, worked with pregnant clients, or spent a lot of time with a loved one/partner who’s pregnant, you’ll realize that fasting is usually a mismatch for pregnancy. If you’ve not had any experiences of the above, then you (probably) won’t know this.
Let me explain, trimester by trimester, from a mindful eating and physiological perspective. Then I’ll go into some research.
First Trimester & Intermittent Fasting: It is even possible?
Say you find out you’re pregnant. The earliest this can happen is ~2 weeks after ovulation or ~4 weeks after your last menstrual period, so you’re already 4 weeks along by the time you know.
By weeks 6-8, nausea and food aversions usually begin thanks to massive—and necessary—hormonal shifts happening to help you sustain the pregnancy, provide fuel for the rapidly dividing cells (all of baby’s internal organs have developed their basic structure by 8 weeks), and support your thyroid as it adapts to provide a huge boost to your metabolism (and support baby’s early brain development).
Your blood sugar also tends to dip in the first trimester as insulin sensitivity temporarily improves and your pancreas starts to grow and produce more insulin (the big surge in insulin production happens later in pregnancy, but this is accompanied by insulin resistance—not so in the first trimester!).
Even for those with type 1 diabetes, who DO NOT otherwise produce insulin, their insulin dosages often drop temporarily in the first trimester (it’s theorized that maybe the beta cell expansion of pregnancy might allow them to produce *some* insulin and/or the temporary drop in insulin resistance reduces the need for exogenous insulin).
What does all this mean?
In the first trimester, you’re probably hungry, but too nauseous to eat much at one sitting (annoying, I know! #solidarity). To settle your stomach, you probably want to eat more carbs, which don’t keep you full for very long, meaning you’re hungry again relatively quickly.
Some people would just tell you to “power through” and eat more fat/protein, which are more satiating, but nausea/food aversions usually mean large portions of those foods don’t sit well or are entirely unappealing.
Your stomach might be so unsettled, unsatisfied—confused, maybe?— that if you wake up in the middle of the night with an empty stomach, your nausea will get even worse. The only way to settle it and get back to sleep? Probably to eat something! (FYI – listening to your body is a wise choice. I kept snacks at my bedside so I didn’t have to trek to the kitchen.)
What’s the solution to all of this?
EAT—and eat as frequently as you need to while you’re in survival mode. You might be eating tiny bites whenever you’re able. It might be 8x per day. It might be 18x per day. Don’t freak out!
Eat whatever keeps you going (more detail on nutritional strategies to manage nausea are explained in Ch 7 of Real Food for Pregnancy).
As someone who relatively recently went through the first trimester again (second pregnancy), let me tell you… intermittent fasting was entirely unimaginable in that phase. HARD NO. That would have gone against every single message my body was sending me. I often had to eat right before bed, sometimes have a snack in the middle of the night, and most days eat something before even getting out of bed.
I’ve worked with HUNDREDS of pregnant clients one-on-one and hear from literally THOUSANDS of mamas via email/messages. My experience of the first trimester isn’t an anomaly. This is normal.
There is absolutely no way I would have been able to eat enough food to sustain myself by compressing my “eating window” into 8 hours. 100% impossible. So why would I try to force it?
Second Trimester & Intermittent Fasting: Maybe, but maybe not.
Some women have a bit of a respite in the second trimester, where nausea and aversions fade (for me, they gradually dissipate by week 15-16, but it’s highly highly highly individual).
This means that for some, a return to a more balanced diet that’s less heavy on the carbs happens naturally. You may notice that you can get through a larger meal, eat enough fat/protein/fiber to stay satisfied for longer, and generally feel a little less urge to eat quite as frequently.
That said, I still don’t think it makes sense to arbitrarily limit your eating window to 8 hours. Your primary nutritional goal in pregnancy should be to meet your micronutrient needs first and foremost. The less time you “allow” yourself to eat, the fewer opportunities you get to eat those micronutrients.
If, by chance, you’re a person who naturally and truly feels GOOD eating two larger meals and you’re able to meet your calorie, protein, and micronutrient requirements with something that resembles intermittent fasting without having to force it (meaning, you’re not hungry during this “fasting window”), then by all means, keep on keepin’ on.
For what it’s worth, I have yet to meet that person. However, it is theoretically possible, so I don’t want to dismiss this entirely. The key is that it isn’t forced, but rather a pattern that your body naturally falls into when you’re following mindful eating cues.
Third Trimester & Intermittent Fasting: Are you kidding me?
By the third trimester, your belly is typically so large that it compresses your stomach, making large meals a no-go.
This stage of pregnancy is when caloric, protein, and micronutrient needs are at their highest.
Can you feasibly eat all 2200-2600 calories that you need within an 8 hour window without experiencing heartburn or extreme abdominal distension? Let me tell you, your abdomen feels distended enough with baby taking up so much room!
What about protein? Data from a 2015 study—the first ever to directly estimate protein requirements in pregnant women—found that protein needs are actually 73% higher than the current estimated average requirement—in late pregnancy. That means a person who weighs about 150# pre-pregnancy requires over 100g of protein per day in the third trimester. That’s a lot of protein to fit into the, say, 2 meals and 1 snack you can reasonably fit in an 8 hour window while still meeting your other nutrient needs and not feel like your belly is going to burst.
What about your blood sugar? Insulin resistance is at its peak in this stage. High blood sugar readings taken one hour after a meal are predictive of excessive fetal growth. It’s one of the reasons that dietary strategies for gestational diabetes involve splitting up meals and snacks throughout the day. Sure, if you’re eating lower carb, you can minimize these spikes, but even as a person with excellent blood sugar metabolism, large meals spike my blood sugar much more than smaller ones.
Some would argue that given the data on intermittent fasting in men shows improvement in blood sugar metabolism that the same should hold true in pregnancy, but as someone who’s worked extensively with gestational diabetes, I can tell you it’s just not necessary.
The vast majority of my clients have excellent blood sugar readings with diet alone while eating 5-6x per day (the lower carb, nutrient dense diet outlined in Real Food for Gestational Diabetes) and not intentionally “fasting” beyond sleeping at night—and even that is usually 10-12 hours or less between dinner/bedtime snack and breakfast.
Are There Studies on Intermittent Fasting & Pregnancy?
Not many. Most of the studies are in relation to religious fasting, like the practice of Ramadan in the Muslim faith.
In case you’re not familiar, “Ramadan is a period during which food and fluid ingestion is not allowed for Muslims between pre-sunrise and post-sunset hours for one month every year.” The fast is broken each evening with one large meal after sunset.
So far, the data on the effects of Ramadan on pregnancy outcomes and infant/child health is mixed. Some data looking solely at pregnancy do not find significant impacts on fetal development (such as fetal movement, oxidative stress, uterine arterial blood flow, and birthweight), while others looking at long term health of the offspring have found some concerning findings (such as lack of stature in adolescence and lower scores on cognitive tests if mothers participated in fasting during Ramadan). Worryingly, some research has shown an increased risk of mortality among children under age 5 years if their mothers participated in Ramadan fasting during the first trimester.
One study showed “significant alteration in the frequency and pattern of human fetal breathing movements” during Ramadan fasting at 30 weeks of pregnancy. It’s unknown what significance this has, but raises concern.
Another, which looked at the effects of Ramadan fasting in 168 women compared to 156 women who did not participate in fasting found that participation in fasting resulted in a significantly higher rate of gestational diabetes, labor induction, C section delivery, and frequency of admission to the neonatal intensive care unit. This is in contrast to a more recent study, which found lower rates of gestational diabetes among women who participated in Ramadan fasting in pregnancy. Abdominal visceral fat mass also appears to decrease among women who participate in Ramadan fasting.
A large review of studies on Ramadan fasting and pregnancy published in 2018 looked at 22 studies encompassing 31,374 pregnancies (including 18,920 pregnancies exposed to Ramadan fasting). Birth weight and risk of preterm delivery were not significantly affected by maternal fasting, however, placental weight was significantly lower in fasting mothers. The significance of this finding is unknown, however some studies suggest low placental weight is associated with malnutrition, poor transfer of nutrients to the fetus, and higher rates of adverse fetal outcomes.
The researchers conclude:
“Ramadan fasting does not adversely affect birth weight although there is insufficient evidence regarding potential effects on other perinatal outcomes. Further studies are needed to accurately determine whether Ramadan fasting is associated with adverse maternal or neonatal outcome.” (BMC Pregnancy and Childbirth, 2018)
What does this research mean?
We really don’t know that much about the effects of intermittent fasting during pregnancy. The data is mixed. Ultimately, this is yet another one of those cases where you need to use personal and clinical judgement.
If your choice to fast is a religious one, that’s up to you.
If your choice to intermittent fast in pregnancy is based on health reasons, I don’t think we have enough evidence to say that it’s guaranteed to be healthier (or even a risk-free choice). Based on my clinical experience and the high prevalence of micronutrient deficiencies among pregnant women (it’s estimated that 47% are deficient in at least one micronutrient in the United States), my opinion is that intermittent fasting is a mismatch for pregnancy.
Although this represents an extreme example, the Dutch famine of 1944-1945, during which food rations were below 1,000 kcal/day for a period of 5 months, showed us that children exposed to this in utero experienced a higher rate of chronic disease later in life. While this arguably does not represent intermittent fasting, but rather starvation, it does give you pause at what the long term consequences could be of under nutrition in pregnancy.
Is it really wise to intentionally minimize our food/nutrient intake at a time when so much is at stake?
Beyond the overnight fast that you naturally take every night while you sleep (typically 12 hours or less between meals), “forcing” your body to go without food during pregnancy is probably not the best call.
Why Intermittent Fasting is a Mismatch for Pregnancy
All in all, most of the typical arguments for intermittent fasting don’t make sense in the context of pregnancy. This is naturally a time when your body very intentionally accrues body fat, gains weight, and preferentially directs nutrients to the embryo/fetus to support rapid growth.
On one of the podcast interviews I appeared on, the host likened an optimal diet in pregnancy to a body building diet. In some ways, this makes a lot of sense. You’re literally growing a human from scratch—that’s certainly counts as body building.
To summarize, here are the reasons you probably don’t want to intermittent fast in pregnancy:
- Micronutrient and calorie needs are high – Intentionally limiting your food intake to a small window can make it difficult to meet these needs.
- Protein needs are high – This is the most filling and satiating macronutrient. It’s going to be very hard to meet those needs in a short eating window, especially in early or late pregnancy.
- Early satiety – In late pregnancy, lack of space in your stomach can make it hard to eat large portions at a time without experiencing heartburn or digestive discomfort.
- Nausea/food aversions – Do I even need to explain? Both of these make small/frequent meals (grazing) more appealing than big meals. If you’ve never been pregnant, imagine how you feel when you get food poisoning or severe motion sickness. It’s that—queasiness often coming in waves, often lasting for hours, days, or months at a time. Large, infrequent meals typically worsen these symptoms.
- Metabolism – Your body wants you to gain weight and grow a baby. This is not the time to be thinking about “autophagy” or avoiding gaining weight/fat mass. These hormonally induced changes in body composition happen for a reason. Don’t fight it. Data from adult men do not necessarily apply to pregnancy. (If you believe your prepregnancy weight puts you in a category where you’d be better off limiting weight gain, please review the section on weight gain in Ch 7 of Real Food for Pregnancy. There is a ton of nuance to this topic and weight goals require a personalized approach.)
- Lack of research – The data on intermittent fasting in pregnancy is mixed. We can’t draw strong conclusions at this time, but there’s enough concerning data available to make you pause.
All told, when someone asks me about intermittent fasting and pregnancy, what I really want to ask is: WHY?
I have yet to hear an argument in favor of intermittent fasting and pregnancy that has robust data to back up the logic. That said, I’ll eagerly read any research studies you come across on the topic!
I fully recognize we don’t have all the answers at this time and I’m always willing to adjust my recommendations as new data becomes available.
Until then, if you’re pregnant, keep eating nutrient-dense real food at whatever interval is working for you. When so much is unknown about intermittent fasting and pregnancy, default to prioritizing micronutrients and honoring your body’s mindful eating cues.
Until next week,
P.S. – On a related note, if you have questions about weight gain in pregnancy, particularly if you’re a practitioner who counsels clients on this sensitive topic, I recommend you check out the recent Deep Dive Discussion webinar on Challenging Counseling Questions that I recently co-hosted via the Women’s Health Nutrition Academy.
The main questions we tackled were:
- How do I best counsel a vegan/vegetarian client on prenatal nutrition?
- What do I do with clients struggling with body image and weight before and during pregnancy?
- How do I set realistic expectations for the timeline and results with my infertility clients?
This webinar is available for no cost. Yep, 100% FREE. Check it out here. Registered dietitians earn 1 CEU for attending.
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- Dikensoy, Ebru, et al. “Effect of fasting during Ramadan on fetal development and maternal health.” Journal of Obstetrics and Gynaecology Research 34.4 (2008): 494-498.
- Karimi, Seyed M., and Anirban Basu. “The effect of prenatal exposure to Ramadan on children’s height.” Economics & Human Biology 30 (2018): 69-83.
- Majid, Muhammad Farhan. “The persistent effects of in utero nutrition shocks over the life cycle: Evidence from Ramadan fasting.” Journal of Development Economics 117 (2015): 48-57.
- Ozturk, Ebru, et al. “Effect of Ramadan fasting on maternal oxidative stress during the second trimester: a preliminary study.” Journal of Obstetrics and Gynaecology Research 37.7 (2011): 729-733.
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- Safari, Kolsoom, Tiran Jamil Piro, and Hamdia Mirkhan Ahmad. “Perspectives and pregnancy outcomes of maternal Ramadan fasting in the second trimester of pregnancy.” BMC pregnancy and childbirth 19.1 (2019): 128.
- Gur, E. B., et al. “Effect of Ramadan fasting on metabolic markers, dietary intake and abdominal fat distribution in pregnancy.” Hippokratia 19.4 (2015): 298.
- Glazier, Jocelyn D., et al. “The effect of Ramadan fasting during pregnancy on perinatal outcomes: a systematic review and meta-analysis.” BMC pregnancy and childbirth 18.1 (2018): 421.