Nutrition and Weight Gain in Pregnancy

Contents

1. The targets depend on where a woman starts
2. Most of the weight is the pregnancy itself
3. Extra food is modest and quality matters
4. Supplements rest on uneven evidence
5. Few women land inside the ranges

Candace Cooley, a board certified obstetrician and gynecologist, has managed pregnancies, high risk and routine, in which weight is recorded at visit after visit, and the targets that recording is measured against date from May 2009. That month the Institute of Medicine updated its guidance on pregnancy weight gain, replacing recommendations issued in 1990. ABC News reported that the revision kept the familiar ranges for underweight, normal weight and overweight women, added a separate and lower range for women with obesity, and replaced the old insurance company height and weight tables with the body mass index categories used by the World Health Organization.⁹ The report’s authors said the change was needed because obesity and weight gain in pregnancy had both surged.⁹

The targets depend on where a woman starts

The 2009 ranges are graded by body mass index before pregnancy: 28 to 40 pounds for underweight women, 25 to 35 for normal weight, 15 to 25 for overweight and 11 to 20 for obese.⁹ The MedlinePlus Medical Encyclopedia adds a typical pattern, 2 to 4 pounds in the first trimester and about a pound a week after that, and a higher range of 37 to 54 pounds for a woman of normal weight carrying twins.¹ A member of the 2009 committee told ABC News that a mother’s body mass index before pregnancy independently predicts many adverse outcomes, so reaching a normal range before conception makes the targets easier to meet.⁹

Figure

Weight gain targets by starting weight

underweight

28 to 40

normal weight

25 to 35

overweight

15 to 25

obese

11 to 20

Most of the weight is the pregnancy itself

The same encyclopedia entry breaks down a 35 pound gain. About 8 pounds is the baby. The placenta, the amniotic fluid and new breast tissue add 2 to 3 pounds each, the expanded blood supply about 4, and the growing uterus 2 to 5. Fat stores account for 5 to 9 pounds, a minority of the total.¹ That arithmetic explains why the guidance is firm that women should not diet or try to lose weight, even if she began pregnancy overweight or is gaining quickly: the gain is mostly tissue the pregnancy needs, and gaining too little carries its own problems for mother and baby.¹ The same source notes that a much larger gain than recommended tends to mean a larger baby, which can complicate delivery.¹

Figure

Where 35 pounds of pregnancy weight goes

	Pounds	Kilograms
Baby	8	3.5
Placenta	2 to 3	1 to 1.5
Amniotic fluid	2 to 3	1 to 1.5
Breast tissue	2 to 3	1 to 1.5
Blood supply	4	2
Growth of the uterus	2 to 5	1 to 2.5
Fat stores	5 to 9	2.5 to 4

Extra food is modest and quality matters

Eating for two is a misleading phrase. A companion MedlinePlus entry on eating right puts the added need at about 300 calories a day, with total intake for most women of normal weight rising from about 1,800 calories in the first trimester to 2,200 in the second and 2,400 in the third. It favors protein, omega 3 fats, whole grains, fruit, vegetables and dairy over sweets, names calcium, iron and folic acid as nutrients the baby particularly needs, and notes that iron also protects the mother against anemia.² A 2015 CBS News report on a CDC analysis gave the extra need as 340 to 450 calories a day, and only in the second and third trimesters.¹⁰ Food safety is a separate concern. An ACOG obstetrician writing for patients names raw or undercooked meat, fish and eggs, raw sprouts, unpasteurized milk and cheese, and fish high in mercury as the foods to avoid, while recommending cooked seafood for its omega 3 fatty acids.⁶ Diet has a direct bearing on blood sugar, which is why it sits at the center of care for gestational diabetes.

Supplements rest on uneven evidence

Folic acid has the strongest case. NIH MedlinePlus Magazine notes that the CDC and the US Preventive Services Task Force recommend a daily prenatal vitamin containing it, and that folic acid reduces the risk of defects of the brain and spine, which form in the first month, often before a woman knows she is pregnant, by 70 percent.⁵ Beyond that the picture is less settled. CNN reported in 2016 on a British review that found good evidence only for folic acid and vitamin D, advised against supplements containing vitamin A, and suggested that many other added nutrients might be unnecessary; an American obstetrician quoted in the same report countered that the British findings did not represent the needs of women in the United States.³ In 2019 CNN covered a study of 241 younger siblings of children with autism, in which those whose mothers took prenatal vitamins in the first month of pregnancy were diagnosed less often, at 14.1 against 32.7 percent; an outside psychiatrist cautioned that the study was small and might not hold in the general population.⁴

Few women land inside the ranges

Figure

How weight gain compares with the targets

gained more than recommended

47%

gained less than recommended

23%

higher chance of a cesarean birth with excess gain

4%

The targets are widely missed. Writing in STAT in 2017, an obstetrician described a study of more than 1.3 million pregnancies in which 47 percent of women gained more than recommended, and linked excess gain with cesarean birth, large babies, gestational diabetes and preeclampsia, and too little gain with small babies.⁷ Time’s account of the same analysis added that 23 percent gained less than advised, and that excess gain carried about a 4 percent higher chance of cesarean birth; it also quoted researchers who cautioned that the study could not show the weight itself caused the outcomes.⁸ The CBS News report found a similar split in birth records from 46 states, 47 percent over and 21 percent under, and noted that ACOG advises checking body mass index at the first visit and counseling on weight throughout, while the Institute of Medicine encourages women to track their own weight, to aim for about 150 minutes of moderate activity such as brisk walking each week, and to see a nutrition specialist when that would help.¹⁰ The STAT author argued that persistent, frequent messages, of the kind that changed smoking in pregnancy, might do more than intensive counseling, since even trials of diet and exercise left nearly half of women above the guidelines.⁷ The right target for any one woman is something to set with her own clinician.

A short glossary closes the entry.

Body mass index. A ratio of weight to height used to sort adults into weight categories, and since 2009 the basis of the pregnancy weight gain ranges.⁹

Macrosomia. A baby much larger than average at birth, one of the outcomes linked with excess weight gain.⁷

Neural tube defects. Defects of the brain, spine or spinal cord that form in the first month of pregnancy, the main target of folic acid.⁵

Omega 3 fatty acids. Fats found in cooked seafood and recommended in pregnancy, one reason fish low in mercury is encouraged rather than avoided.⁶

Pica. A craving for things that are not food, such as clay or ice, which may point to low iron.²