Fetal weight gain depends heavily on how well the placenta delivers nutrients and oxygen, so “helping” your baby grow is really about supporting placental function and removing obstacles to it. Some of those obstacles are within your control: what you eat, whether you smoke, how you manage stress. Others, like genetic conditions or the structure of the placenta itself, are not. The good news is that the controllable factors genuinely matter, and a few medical interventions can make a measurable difference when a baby is falling behind.
Why Some Babies Don’t Grow as Expected
When a baby isn’t reaching the weight your provider expected, the most common culprit is the placenta. The placenta is the baby’s lifeline, ferrying oxygen, glucose, amino acids, and everything else the fetus needs. When blood flow through the placenta is compromised, the baby gets short-changed on the raw materials for growth. This is called placental insufficiency, and it underlies most cases of fetal growth restriction.
Placental insufficiency often traces back to a problem that starts very early in pregnancy: the blood vessels in the uterine wall fail to remodel properly to accommodate the high-volume blood flow a growing fetus demands. When this remodeling is incomplete, the placenta is chronically underperfused, sometimes subtly enough that it only becomes apparent in the third trimester when fetal demand peaks.1American Journal of Obstetrics and Gynecology. Placental-derived fetal growth restriction Emerging research describes this not as a single defect but as a cascade of interconnected problems, including oxidative stress, immune dysregulation, and disrupted nutrient transport, all reinforcing one another.2PubMed Central. Placental dysfunction drives fetal growth restriction: mechanisms and translational perspectives
Not every small baby has a problem, though. Some babies are simply small because their parents are small. The distinction between a baby who is constitutionally small for gestational age and one who is growth-restricted matters clinically, because the growth-restricted baby is the one at risk for complications. Providers use ultrasound measurements, Doppler blood flow studies, and sometimes blood markers to tell these apart.3Reproductive, Female and Child Health. Evaluation of the HALP Score in the Distinction Between Fetal Growth Restriction and Small for Gestational Age
Nutrition That Actually Moves the Needle
If you’ve been told your baby is small, the instinct to “eat more” is understandable, but the type of nutrition matters more than the quantity. The most consistent evidence points to balanced protein and energy intake. A meta-analysis of supplementation trials found that giving pregnant women balanced protein-energy supplements increased birthweight by roughly 60 grams on average and reduced the likelihood of having a low-birthweight baby by about a quarter.4PubMed Central. Effect of balanced protein–energy supplementation given to pregnant women on birth outcomes: A systematic review and meta-analysis In populations where women were nutritionally deficient to begin with, the gains were larger, with birthweight increases in the range of 95 to 324 grams.5PubMed Central. Effects of protein energy supplementation during pregnancy on fetal growth: a review of the literature focusing on contextual factors
The word “balanced” is doing real work in that sentence. Protein is essential for fetal growth, but too much protein can actually backfire. Extremely high-protein diets during pregnancy have been linked to growth restriction and even pregnancy loss, because excessive amino acid breakdown produces toxic byproducts like ammonia and homocysteine.6PubMed Central. Impacts of maternal dietary protein intake on fetal survival, growth, and development The practical message: aim for adequate protein from varied sources (meat, fish, eggs, legumes, dairy) without going overboard with protein shakes or extreme high-protein regimens. Supplements marketed to pregnant women that contain protein should derive no more than about 20 percent of their calories from protein to stay in the “balanced” range that the evidence supports.5PubMed Central. Effects of protein energy supplementation during pregnancy on fetal growth: a review of the literature focusing on contextual factors
Folic acid also plays a role beyond its well-known importance for preventing neural tube defects. A systematic review of studies in low- and middle-income settings found that folic acid supplementation was associated with an average birthweight increase of about 370 grams compared to controls, and a roughly 40 percent reduction in the odds of low birthweight.7PubMed Central. Maternal folic acid supplementation and infant birthweight in low‐ and middle‐income countries: A systematic review That effect was likely amplified because many women in those settings were folate-deficient to begin with, so the takeaway for someone already taking a prenatal vitamin with adequate folate is more modest. Still, if you aren’t consistently taking a prenatal vitamin, starting one is one of the simplest and most evidence-backed steps you can take.
Quitting Smoking Is the Single Biggest Lifestyle Change
If there is one modifiable factor that dwarfs everything else, it’s cigarette smoking. Nicotine constricts blood vessels, including the ones feeding the placenta, and the carbon monoxide in cigarette smoke directly competes with oxygen for binding sites on your red blood cells. The result is a baby that gets less oxygen and fewer nutrients, consistently, for months. A large study tracking birthweights across maternal BMI categories found that smokers had significantly lighter babies at every body size, with birth weights ranging from about 2,995 grams in underweight frequent smokers to 3,607 grams in obese nonsmokers. The differences between smokers and nonsmokers were highly significant across every weight category.8PubMed. Impact of nicotine and maternal BMI on fetal birth weight
Quitting at any point during pregnancy helps. The earlier you quit, the more time the placenta has to recover some of its normal blood flow. If you’re struggling, talk to your provider about safe cessation strategies during pregnancy, because this is the intervention with the largest proven effect on fetal weight among things you can directly control.
Sleep Position in Late Pregnancy
This one surprises many people: how you lie down in the third trimester can meaningfully change blood flow to your uterus. MRI studies have shown that lying flat on your back (supine) in late pregnancy reduces blood flow to the uterus through the internal iliac arteries by about 24 percent and cuts oxygen transfer across the placenta by roughly 6 percent, compared to lying on your side.9PubMed Central. The effects of maternal position, in late gestation pregnancy, on placental blood flow and oxygenation: an MRI study The reason is mechanical: the weight of the pregnant uterus compresses major blood vessels, particularly the vena cava, when you lie on your back.
Sleeping on your side, especially your left side, keeps those vessels uncompressed and maximizes flow to the placenta. You don’t need to obsess about waking up on your back at 3 a.m.; the concern is mainly about prolonged supine positioning. A pillow wedged behind you or between your knees can help you stay on your side through the night. For a baby that’s already small, every bit of placental blood flow counts, and this adjustment costs nothing.
Exercise During Pregnancy
There is a persistent worry that exercise diverts blood away from the baby and could stunt growth. The reality is more nuanced. During a bout of exercise, blood flow to the uterus does temporarily dip as your muscles demand more oxygen. But in women who exercise regularly, the body adapts: resting blood volume and cardiac output increase, and placental function improves to compensate.10PubMed. The effects of maternal exercise on fetal oxygenation and feto-placental growth
Moderate exercise, like walking, does not appear to harm fetal growth. A randomized trial of previously sedentary pregnant women found that a moderate-intensity walking program improved the women’s physical fitness without any difference in birthweight or placental blood flow compared to women who didn’t exercise.11Obstetrics & Gynecology. Effect of a Physical Exercise Program During Pregnancy on Uteroplacental and Fetal Blood Flow and Fetal Growth Starting a moderate exercise routine may even improve placental development and slightly increase birthweight, though maintaining a very rigorous routine throughout pregnancy can selectively reduce fetal fat stores.10PubMed. The effects of maternal exercise on fetal oxygenation and feto-placental growth If you’re concerned about a small baby, moderate activity is safe and likely beneficial, but this is not the time to train for a marathon.
Stress and Cortisol
Chronic stress isn’t just hard on you; it reaches the baby through hormonal pathways. Mothers of small-for-gestational-age babies report significantly higher perceived stress and anxiety than mothers of appropriately grown babies. Research has shown that in these pregnancies, the placenta’s ability to buffer cortisol is reduced, which alters the cortisol metabolism in the fetus’s amniotic fluid.12PubMed Central. Maternal stress and fetoplacental cortisol regulation in small-for-gestational-age newborns The placenta normally acts as a gatekeeper, breaking down the stress hormone cortisol before it reaches the fetus. When that gatekeeper function is weakened, the baby is exposed to more cortisol, which can constrain growth.
Telling a pregnant person to “just relax” is unhelpful and often dismissive. What is useful is recognizing that chronic, sustained stress is a genuine biological input, not just an emotional experience. Practical approaches like therapy, social support, reducing work demands if possible, and even mindfulness-based stress reduction programs have all been studied in pregnancy contexts. If your provider has flagged your baby as small and your stress levels are through the roof, addressing the stress isn’t a luxury; it’s part of a rational strategy for supporting fetal growth.
Medical Interventions Your Doctor May Offer
When a baby is genuinely growth-restricted, or when a mother is at high risk for conditions like preeclampsia that impair placental function, several medical interventions come into play.
Low-Dose Aspirin
Low-dose aspirin (typically 75 to 150 mg daily, started before 16 weeks of gestation) is one of the most studied and effective interventions for preventing placental dysfunction. It works by reducing microthrombosis in the small blood vessels of the placenta and lowering vascular resistance. Meta-analyses show that it reduces the risk of fetal growth restriction by about 18 percent, and the benefit extends to preeclampsia prevention as well.13PubMed Central. The Role of Acetylsalicylic Acid in the Prevention of Pre-Eclampsia, Fetal Growth Restriction, and Preterm Birth In one trial of high-risk women, fetal growth restriction occurred in only about 4 percent of those taking aspirin versus 19 percent of controls.14Scientific Reports. Early initiation of low-dose aspirin for the prevention of pre-eclampsia in high-risk pregnancies The catch: timing matters. Starting aspirin after 16 weeks appears to be significantly less effective, so this is a conversation to have early in pregnancy if you have risk factors like a history of preeclampsia, chronic hypertension, or prior growth-restricted pregnancies.
L-Arginine
L-arginine is an amino acid that serves as the raw material for nitric oxide, a molecule that relaxes blood vessels. In pregnancies already complicated by growth restriction, L-arginine supplementation has been shown to improve placental blood flow (measured by Doppler ultrasound), increase birthweight, and improve newborn outcomes in multiple studies.15PubMed. L-Arginine supplementation in pregnancy: a systematic review of maternal and fetal outcomes 16PubMed Central. Effect of l-Arginine on Nitric Oxide Levels in Intrauterine Growth Restriction and its Correlation with Fetal Outcome This isn’t something to self-prescribe; it’s typically offered by a maternal-fetal medicine specialist once growth restriction has been diagnosed and blood flow studies suggest placental resistance is elevated.
Doppler Monitoring
This isn’t a treatment so much as a tool that guides treatment decisions. Umbilical artery Doppler ultrasound measures how much resistance the baby’s blood encounters flowing through the placenta. Rising resistance, absent flow at the end of each heartbeat, or reversed flow each represent progressively more serious placental failure, and they help your provider decide how closely to monitor you and when to deliver the baby.17Lynchburg Journal of Medical Science. Umbilical Artery Doppler in Fetal Growth Restriction If you’ve been told your baby is small, regular Doppler assessments are standard care, and asking about what your Doppler results show is entirely reasonable.
Conditions That Are Harder to Change
Some causes of poor fetal growth aren’t something a diet or lifestyle change can fix, and it’s important to name them so that parents dealing with them don’t blame themselves.
Maternal conditions like preeclampsia directly impair placental function through a combination of inflammation, abnormal blood clotting, and blood vessel spasm. The growth restriction in these cases isn’t a separate problem; it’s a symptom of the same underlying placental dysfunction driving the preeclampsia itself.18PubMed Central. Preeclampsia and Fetal Growth Restriction as Risk Factors of Future Maternal Cardiovascular Disease-A Review Managing blood pressure and delivering at the right time are the primary strategies here.
Genetic factors in the fetus itself can also limit growth. Chromosomal abnormalities, single-gene disorders, and certain epigenetic changes can all constrain how large a baby grows, independent of placental function.19PubMed Central. Genetics Etiologies Associated with Fetal Growth Restriction When growth restriction is detected early and severely, genetic testing is often recommended to look for these causes.
Infections during pregnancy can also sabotage placental growth. Cytomegalovirus, or CMV, is one of the most common congenital infections and can impair the placenta’s development by damaging the cells that anchor it to the uterine wall and disrupting the blood vessel remodeling needed for adequate blood flow.20PubMed Central. The Role of Congenital Cytomegalovirus Infection in Adverse Birth Outcomes: A Review of the Potential Mechanisms 21PubMed. Congenital cytomegalovirus infection undermines early development and functions of the human placenta CMV spreads through saliva and urine, particularly from young children, so basic hygiene measures like handwashing after diaper changes and not sharing utensils with toddlers are practical steps, though they won’t help if infection has already occurred.
Twin Pregnancies and Fetal Weight
Twin pregnancies face unique growth challenges. In twins with separate placentas (dichorionic), growth restriction typically arises from the same placental insufficiency that affects singletons, just potentially affecting one twin more than the other. In identical twins sharing a single placenta (monochorionic), the problem is more specific: the twins share blood vessel connections within the placenta, and unequal distribution of flow can leave one twin significantly smaller than the other.22The Obstetrician & Gynaecologist. Assessment of fetal growth in twin pregnancies: evidence‐based practice Growth surveillance in twin pregnancies is more frequent and uses different charts than singleton pregnancies. If you’re carrying twins and one is falling behind, the management strategy depends heavily on the type of twin pregnancy you have.
Altitude and Fetal Growth
This one rarely comes up in prenatal advice, but it’s surprisingly well-documented. Living at high altitude reduces the amount of oxygen available in every breath you take, and the placenta isn’t immune to this. A large review of studies found that for every 1,000-meter increase in altitude, average birthweight drops by about 91 grams, and the prevalence of low birthweight climbs significantly.23PubMed Central. Cause of fetal growth restriction during high-altitude pregnancy For someone living at sea level, this is irrelevant. But if you live above 2,500 meters and your baby is measuring small, altitude is a genuine contributing factor. Populations that have lived at high altitude for many generations (like Andean and Tibetan communities) show some biological adaptation to this effect, but newcomers to altitude do not. There is no practical fix beyond being aware that it’s a factor and ensuring all other modifiable risks are minimized.
Why Fetal Growth Matters Beyond Birth
The stakes of fetal growth restriction extend well beyond the delivery room. The “fetal programming” hypothesis, supported by decades of research, holds that the conditions a fetus experiences in the womb set the baseline for metabolic and cardiovascular function into adulthood. A baby whose growth was restricted in utero adapts to a nutrient-poor environment by reprogramming its metabolism in ways that become maladaptive after birth, when nutrition is typically abundant. This mismatch is thought to increase the risk of cardiovascular disease, type 2 diabetes, and metabolic syndrome later in life.24PubMed Central. What is fetal programming?: a lifetime health is under the control of in utero health This doesn’t mean every small baby is destined for chronic disease. It means that supporting adequate fetal growth, to whatever extent is possible, has implications that reach far beyond the number on the scale at birth.
For parents worried about a small baby, the knowledge that long-term health outcomes are also at play can feel frightening. But it cuts both ways: it validates taking the issue seriously and pursuing every reasonable intervention, and it also underscores why close follow-up after birth matters. Growth-restricted babies who receive good nutrition and medical monitoring in infancy often show robust catch-up growth, which can partially offset the metabolic reprogramming that occurred in utero.
Blood Sugar Control in Gestational Diabetes
Gestational diabetes is often discussed in the context of babies getting too big, but poorly managed blood sugar can work in both directions. In women with gestational diabetes, the relationship between blood sugar levels, maternal weight gain, and fetal size is complex. Research has found that glycemic control, weight gain, and the type of treatment used all independently predict whether a baby ends up too large or too small for gestational age.25PubMed. Gestational diabetes: maternal weight gain in relation to fetal growth, treatment modality, BMI and glycemic control In cases where blood sugar is aggressively managed with insulin, there’s a real risk of overrestricting fetal glucose delivery. Conversely, women with gestational diabetes who also have preeclampsia or placental insufficiency can end up with growth-restricted babies despite the metabolic profile that would normally push toward larger size. If you have gestational diabetes and your baby is measuring small, your provider should be looking at the placenta and not just the glucose numbers.