In the course of pregnancy you will build another human being on some occasions even more than one. To equip your body to do that, to nurture and deliver the baby, and to feed it for months afterwards, your body needs to add new tissue. The first new additions are the tissues of the baby itself, the umbilical cord, the placenta, and the amniotic fluid. Second is the growth of the maternal organs directly involved in pregnancy the uterus, the breasts, and the solid and fluid parts of the circulating blood. Next is the increased fluid that accumulates within body tissues. Finally, the addition of fat is a small part of the process.
Over the years, recommendations about weight gain in pregnancy have followed a sweeping pendulum. Before 1970, women were limited to gaining twenty pounds, or at most twenty five pounds. Stepping on the scale at prenatal visits was dreaded by many women whose natural appetite led to higher weight gains. Low weight gain was believed to reduce the problems associated with the birth of large babies and help prevent toxemia of pregnancy. Toxemia, is a pregnancy illness marked by rapid weight gain and swelling, along with high blood pressure and protein in the urine. We now know that the in creased weight gain is the consequence of toxemia, not its cause.
In the 1970s, research began to show that restricted, rather than excess, weight gain could lead to problems for the developing fetus. Low weight gain was found to be associated with lowered birth weight in newborns, a cause of newborn illness and death. Starting in the 1970s, women were told to gain approximately twenty-four pounds, with a range of twenty to twenty five pounds. The pattern of weight gain also received attention at that time. Less than two pounds per month was considered inadequate and more than six pounds per month, excessive. These limitations were still difficult for many pregnant women.
A new approach to pregnancy weight gain was advocated in 1990, following the publication of an extensive report by the institute of Medicine of the National Academy of Sciences. To determine optimal weight gain, this group studied a large number of births. They assessed the range of weight gain associated with what they defined as a favorable pregnancy out come birth between 39 to 41 weeks gestational age and birth weight of 6.6 to 8.8 pounds.
In this new approach, widely accepted today, recommended weight gain varies according to whether the woman is normal weight, under weight, overweight, or obese. These categories are based on a calculation of a woman’s prepregnant weight for height. This calculation is also called the BMI or body mass index. Once a woman’s BMI is calculated, it is compared to a cutoff score and the woman is assigned into one of the four weight categories. Within each weight group, the recommendations span a range of ten to fifteen pounds. With these guidelines, pregnant women should no longer go hungry or feel guilty for gaining weight.
The BMI is calculated by dividing weight (in kilograms) by height (in centimeters) squared. Your physician or midwife should have a table of BMIs so your BMI can be determined easily at your first pregnancy visit or preconceptional visit. Underweight women are those whose weight for height is <90 percent of the normal weight, overweight is 120 to 135 percent of the normal; obese is > 135 percent of the normal weight. The adult female cutoff weights for the four categories are shown in the table.
These weight gains are meant to create the most likely environment for the best possible outcome for the infant. As African-American infants weigh less at any given gestational age, the institute of medicine recommends that the weight gain for african-american women be at the upper end of the range. This is also true for infants of very young women, those who have been menstruating for less than 2 years at the time of their pregnancy. Very short women (<5’2″) should keep their weight gain to the lower end of the normal range, even a few pounds less is considered acceptable for short women. While the minimum weight gain for obese women is fifteen pounds, obese women may gain. less than fifteen pounds without adverse effects on the newborn. Women should never lose weight during pregnancy, regardless of their pre-pregnant weight. The recommended total weight gain for women carrying twins is thirty five to forty five pounds.
For women whose weight falls within the normal range, the recommended pattern of gain is one pound per week during the second and third trimesters. Most women gain only a few pounds (two to five pounds) during the first trimester. Women who suffer from nausea and vomiting may gain even less during the first trimester without untoward effects on the pregnancy or fetus. In the second and third trimesters underweight women should try to gain slightly more than a pound per week and overweight women should gain about two thirds of a pound per week. Of course, no one weekly reading is able to indicate poor or excessive weight gain. Some women show slightly more erratic patterns but over time gain appropriately.