Hormones of Pregnancy

There are numerous steroid hormones, peptide hormones, and prostaglandins influencing the course of pregnancy. Some of them, such as the placental hormones human placental lactogen and human growth hormone, are produced only during pregnancy. Others, including insulin, glucagon, and thyroxine, are present in altered amounts compared with the nonpregnant state and have profound influences on metabolism throughout gestation.

Progesterone and estrogen have a particularly strong influence on pregnancy. The action of progesterone promotes development of the endometrium (mucous membrane of the uterus) and relaxes the smooth muscle cells of the uterus. This relaxation serves to both help the uterus expand as the fetus grows and prevent any premature contractions of the uterus. The same effect also influences other smooth muscle cells, such as the gastrointestinal (GI) tract. The resulting slowing of the GI tract during pregnancy may increase the absorption of several nutrients, most notably iron and calcium. One perhaps annoying consequence of this decreased gut motility is the promotion of constipation. Progesterone causes increased renal sodium excretion during pregnancy. The body compensates for this sodium-losing mechanism by increasing aldosterone secretion from the adrenal gland and renin from the kidney. Sodium restriction during pregnancy, once thought to prevent hypertensive disorders of pregnancy, is actually harmful because it reduces plasma volume and cardiac output.

Estrogen promotes the growth of the uterus and breasts during pregnancy and renders the connective tissues in the pelvic region more flexible in preparation for birth.

Metabolic Changes

Profound changes in maternal metabolism occur during pregnancy, and successful adaptation to these changes is necessary for a favorable pregnancy outcome. The basal metabolic rate (BMR) rises during pregnancy by as much as 15% to 20% by term. This increase is caused by the increased oxygen needs of the fetus and the maternal support tissues. There are alterations in maternal metabolism of protein, carbohydrate, and fat. The fetus prefers to use glucose as its primary energy source. Changes occur in maternal metabolism to accommodate this need of the fetus. The adaptation allows the mother to use fat as the primary fuel source, thus permitting glucose to be available to the fetus.¹ Increased macronutrient and micronutrient intake by the mother during pregnancy ensures that these increased metabolic needs are met.

Anatomic and Physiologic Changes

Plasma volume doubles during pregnancy, beginning in the second trimester. Failure to achieve this plasma expansion may result in a spontaneous abortion, a stillbirth, or a low birth weight infant. One of the results of this increase in plasma volume is a hemodilution effect, or dilution of the blood. In other words, measured components in the plasma such as hemoglobin, serum proteins, and vitamins will appear to be at lower levels during pregnancy because there is a greater volume of solvent (the plasma) relative to concentrations of solute (the components). Cardiac hypertrophy occurs to accommodate this increased blood volume, accompanied by an increased ventilatory rate.

In the kidneys, the glomerular filtration rate (GFR) increases to accommodate the expanded maternal blood volume being filtered and to carry away fetal waste products. As a result of this increase in GFR, small quantities of glucose, amino acids, and water-soluble vitamins may appear in the urine. Although minor losses may be acceptable, a woman who excretes large amounts of protein may experience a more serious problem called preeclampsia, or pregnancy-induced hypertension, which needs strict medical monitoring. Preeclampsia is described in more detail later in the chapter.

As mentioned, progesterone may slow GI motility during pregnancy, leading to constipation, heartburn, and delayed gastric emptying. In late pregnancy, these problems may be exacerbated by the weight of the uterus and fetus as they compress the abdominal cavity.

Weight Gain in Pregnancy

There are three components to maternal weight gain: (1) maternal body composition changes, including increased blood and extracellular fluid volume; (2) the maternal support tissues, such as the increased size of the uterus and breasts; and (3) the products of conception, including the fetus and the placenta. Inadequate weight gain by the mother during pregnancy suggests she may not have received the proper nutrients during pregnancy. Poor weight gain may then lead to intrauterine growth retardation in the infant. Infants born small for gestational age (SGA) or low birth weight are more likely to require prolonged hospitalization after birth or be ill or die during the first year of life. SGA is when an infant is born at a lower birth weight than expected for the length of gestation, while low birth weight is a weight less than 5.5 pounds (2500 g) at birth. Additionally, infant mortality rate, which in part reflects maternal weight gain, is regarded as one measure of a country’s health and well-being. Although the 2007 infant mortality rate for the United States (6.8 per 1000 live births) continued an all-time low first reached in 1996 (6.9 per 1000 live births),² it still remains far greater than other developed countries. Infant mortality rates are higher among non-Hispanic black infants than among non-Hispanic white and Hispanic infants.²

There is strong evidence that the pattern of weight gain is just as important as the absolute recommended weight gains, as shown in Table 11-1. Failure to gain adequately during the second trimester of pregnancy is associated with poor infant birth weight, even if the net gain falls with in the recommendations.

A balance must be struck regarding weight gain during pregnancy. Although women who are underweight or normal weight (as defined by body mass index [BMI]) are counseled to eat sufficiently to promote adequate gain, caution must be observed in counseling women who enter pregnancy overweight or obese. Overweight and obese women should gain enough weight to support the fetus and maternal support tissues but without increasing total body fat. There are increased risks for operative delivery, increased maternal postpartum weight, gestational diabetes, and other long-term health consequences when maternal weight goes beyond the guidelines, particularly among women who are obese before pregnancy.¹ In addition, there may be subpopulations such as minorities and low-income women who need special guidance regarding weight gain during pregnancy. Figure 11-1 summarizes possible determinants and effects on gestational weight gain.

Additional issues arise when women who have underdone gastric bypass surgery become pregnant. Because of smaller stomach size, less food is consumed, and intestinal absorption of nutrients may be compromised. Recommendations are to delay pregnancy until at least a year after bypass surgery and to seek nutrition therapy to support adequate nutrient absorption and energy intake.

Energy

It is difficult to estimate the true energy cost of pregnancy, but the best estimates place the total energy cost somewhere between 68,000 kcal and 80,000 kcal. The increase accommodates the rise in maternal BMR during pregnancy, as well as the synthesis and support of the maternal and fetal tissues.¹ The current recommendation is for a woman to consume an extra 300 kcal per day during the second and third trimesters of pregnancy. Although she is eating for two, the expectant mother need not and should not double her food intake. An extra sandwich and a glass of milk can easily provide the additional 300 kcal per day, providing she was eating well before pregnancy. Personal preference may guide particular food choices to provide the extra kcal, as long as the foods are nutritious.

What happens if a pregnant woman fails to increase her energy intake during pregnancy? The best-known example in the twentieth century occurred in Holland during World War II. Infants born during the famine of 1944 and 1945 had smaller birth weights and birth lengths when compared with infants born either before or after the famine.³ Recent research shows that when women who begin pregnancy in energy deficit (e.g., those who are chronically undernourished in developing countries) are provided with energy supplementation throughout the course of pregnancy, there is a positive effect on maternal weight gain and infant birth weight.⁴ On the other hand, some research suggests that women in the United States who are well nourished do not increase their total energy intake by a full 300 kcal per day and still have a positive pregnancy outcome. Most likely, in the third trimester, many women decrease their energy expenditure in pregnancy by decreasing activity, thereby giving a net increase in energy intake.⁵

Pregnancy is not a time to restrict kcal or to lose weight, even if the mother begins the pregnancy as overweight. This may be particularly important to emphasize to the adolescent population. The mother should be encouraged to eat at least the minimum number of servings recommended during pregnancy from MyPyramid (Box 11-1). The interactive MyPyramid Plan for Moms creates a personalized dietary food pattern based on height, weight, age, and other characteristics. Sample menus can be helpful in showing the pregnant woman how MyPyramid can be used (Box 11-2).

BOX 11-1   Myplate

Pregnancy and Breastfeeding

Health & Nutrition Information for Pregnant & Breastfeeding Women

When you are pregnant or breastfeeding, you have special nutritional needs. This site is designed just for you. It has advice you need to help you and your baby stay healthy.

First – visit your doctor or health care provider if you haven’t already. Every pregnant woman needs to visit a doctor regularly. Only he or she can make sure both you and your baby are healthy. Your doctor can also prescribe a safe vitamin and mineral supplement, and anything else you may need.

Next – get you own Daily Food Plan for Moms. Your Plan will show you the foods and amounts that are right for you. Enter your information for a quick estimate of what and how much you need to eat. Or, enter the foods you eat into the Supertracker to see how your food choices compare to what you need.

Then – learn more by choosing a topic from the menu below. “Sources of information” will take you straight to the government’s best advice on pregnancy and breastfeeding.

Making Healthy Choices in Each Food Group

Follow your Daily Food Plan for Moms and eat the amount recommended for each food group. Include the foods listed below—they are the best sources of some nutrients you need when you are pregnant or breastfeeding.*

Vegetable Group

(choose fresh, frozen, canned, or dried)

• Carrots

• Sweet potatoes

• Pumpkin

• Spinach

• Cooked greens (such as kale, collards, turnip greens, and beet greens)

• Winter squash

• Tomatoes and tomato sauces

• Red sweet peppers

These vegetables all have both vitamin A and potassium. When choosing canned vegetables, look for “low-sodium” or “no-salt-added” on the label.

Fruit Group

(choose fresh, frozen, canned, or dried)

• Cantaloupe

• Honeydew melon

• Mangoes

• Prunes

• Bananas

• Apricots

• Oranges

• Red or pink grapefruit

• 100% prune juice or orange juice

These fruits all provide potassium, and many also provide vitamin A. When choosing canned fruit, look for those canned in 100% fruit juice or water instead of syrup.

Dairy Group

• Fat-free or low-fat yogurt

• Fat-free milk (skim milk)

• Low-fat milk (1% milk)

• Calcium-fortified soymilk (soy beverage)

These all provide the calcium and potassium you need. Make sure that your choices are fortified with vitamins A and D.

Grain Group

• Fortified ready-to-eat cereals

• Fortified cooked cereals

When buying ready-to-eat and cooked cereals, choose those made from whole grains most often. Look for cereals that are fortified with iron and folic acid.

Protein Foods Group

• Beans and peas (such as pinto beans, soybeans, white beans, lentils, kidney beans, chickpeas)

• Nuts and seeds (such as sunflower seeds, almonds, hazelnuts, pine nuts, peanuts, and peanut butter)

• Lean beef, lamb, and pork

• Oysters, mussels, crab

• Salmon, trout, herring, sardines, and pollock

NOTE: Do not eat shark, swordfish, king mackerel, or tilefish when you are pregnant or breastfeeding. They contain high levels of mercury. Limit white (albacore) tuna to no more than 6 ounces per week. Learn more about the safety of eating seafood during pregnancy. (Go to Food Safety at www.choosemyplate.gov for more information about safety of eating fish during pregnancy.)

All of these foods provide protein. In addition, beans and peas provide iron, potassium, and fiber. Meats provide heme-iron -which is the most readily absorbed type of iron. Nut and seeds also contain vitamin E. Seafood provides omega-3 fatty acids.

Accessed June 14, 2012 from http://www.choosemyplate.gov/pregnancy-breastfeeding.html

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^*The foods on this list are the best sources of one or more of the following nutrients: vitamin A, vitamin E, potassium, and iron. Food sources of these nutrients are included because when choosing a typical mix of food choices in each food group, the intake patterns may not meet dietary standards for pregnant and/or breastfeeding women for these nutrients. Accessed June 14, 2012, from http://www.choosemyplate.gov/pregnancy-breastfeeding/making-healthy-food-choices.html.

BOX 11-2

Sample Menu for Pregnant Women

Breakfast

Orange juice	cup
Whole grain toast	1 slice
Banana	medium
Oatmeal	1 cup
Skim milk	1 cup

Midmorning Snack*

Lunch

Roast beef (2 oz lean) sandwich on whole grain bread with lettuce and tomato
Green salad	cups leafy greens, 1 tbsp balsamic vinegar dressing
Orange wedges	1 cup
Skim milk†	1 cup

Midafternoon Snack*

Dinner

Sesame chicken (or fish)	4 oz
Broccoli	cup
Sweet potato	1 medium
Mixed salad (carrots, tomato, cups spinach, romaine lettuce)
Olive oil	2 tsp
Fresh lime juice or vinegar	2 tsp
Whole grain bread	1 slice
Butter	1 tsp
Fresh fruit salad	cup
Skim milk	1 cup

Snack* (Evening if Needed)

Some examples are:

Cereal (ready to eat) cup with skim milk ( cup)

Fresh or frozen berries ( cup) with nonfat yogurt ( cup)

Apple/pear with cheese (1 oz) or peanut butter (1 tbsp)

Fruit and yogurt shake

Open-face peanut butter and apple sandwich

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^*Snacks are all interchangeable

^†Assumes water consumed throughout the day as a beverage in addition to skim milk.

Protein

The Recommended Dietary Allowance (RDA) for protein during pregnancy is 71 g per day for adolescent and adult women. Women can easily obtain this in the American diet; the use of special protein powder supplements is not recommended. Pregnant patients may be counseled to include appropriate sources of protein providing vitamins, minerals, and moderate amounts of fat. Clients from low-income populations may need counseling or other assistance to ensure protein intake is sufficient; these clients may qualify for food vouchers through the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) of the U.S. Department of Agriculture (see the Social Issues box, Providing the Essentials).

The increase in protein intake over the nonpregnant state is necessary to build and maintain the variety of new tissues of pregnancy. A woman experiencing nausea and vomiting in the first trimester of pregnancy may find it difficult to increase sources of protein in her diet, particularly if meats (which have a strong cooking odor) aggravate the nausea. If this is the case, she should consume small amounts of high-quality protein as tolerated.

Vitamin and Mineral Supplementation

The DRIs are increased during pregnancy for most vitamins and minerals. Vitamins of concern are vitamins A and D. While the RDA for vitamin A is 750 to 770 mcgRAE (Retinol Activity Equivalents) preformed vitamin A, the Tolerable Upper Intake Level (UL) is set at 2800 to 3000 mcgRAE preformed vitamin A per day because of the potential for birth defects from excessive intake.⁶ Similarly, excessive vitamin D during pregnancy may cause birth defects so that the Adequate Intake (AI) (5 mcg per day) and UL (50 mcg per day) are the same for women regardless of physiological state.⁶ Micronutrient needs may be met with a balanced diet, with a few notable exceptions including folate and iron. All supplementation during pregnancy should be in the form of prenatal type multivitamin-mineral supplements as recommended by primary health care providers or dietitians.

Caffeine

Whether a woman should refrain from caffeine consumption during pregnancy has been a matter of debate. Caffeine (1-, 3-, 7-trimethyxanthine) may alter deoxyribonucleic acid (DNA) and, in some individuals, may alter circulating levels of neurotransmitters and increase blood pressure.⁷ It has been argued that any or all of these effects may have direct adverse consequences on the developing fetus. However, there is enough evidence suggesting that caffeine is not a human teratogen, and even at modest doses (<300 mg/day or about 2 cups or less of coffee), there is no increased risk of spontaneous abortion or preterm labor. It doesn’t affect birth weight, gestational age, or fetal growth.⁷ There may be small reductions in birth weight at very high levels of consumption. The important issue may be that heavy use of nonnutritive substances such as coffee, tea, and cola may displace needed nutrients in the diet and thus interfere with prenatal development. Moderation of caffeine use during pregnancy as opposed to complete elimination is reasonable advice.

Drugs

A pregnant woman should not consume any over-the-counter or prescription medications unless prescribed by her primary health care provider. The growing fetus, particularly during the period of organogenesis in the first trimester, is highly susceptible to insult.

Although not a direct nutrient concern, the acne medication isotretinoin (Accutane) contains high levels of retinoic acid in the form of a vitamin A analog. This medication causes fetal malformations such as craniofacial abnormalities and microcephaly (abnormal smallness of head with brain underdevelopment) when ingested in the periconceptional period. The current recommendation is that women of childbearing age not use isotretinoin for the treatment of acne.

This recommendation is consistent with a large body of animal data that show that consumption of large quantities of preformed vitamin A during pregnancy results in an excess of malformations such as anencephaly (defective brain development), cleft palate, spina bifida, webbed fingers or toes, and facial malformations. Vitamin A crosses the placenta by simple diffusion. Because it is fat soluble, the excess vitamin A can accumulate in the fetal tissues and may cause damage by interfering with cellular growth and differentiation during critical periods of development.⁸

Alcohol

The use of alcohol during pregnancy may produce fetal alcohol syndrome (FAS) or fetal alcohol spectrum disorder (FASD) in the infant. Symptoms include central nervous systems defects and specific anatomic defects such as a low nasal bridge, short nose, flat midface, and short palpebral fissures (separation between the upper and lower eyelids) (Figure 11-2).

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