Pregnancy Nutrition Surveillance System - PNSS Health Indicators

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Maternal Health Indicators

Prepregnancy Body Mass Index (BMI) is a measure of weight for height expressed as wt (kg) / ht (m2) before the woman became pregnant. The BMI cut-off values specified by the Institute of Medicine (IOM) in 2009 are commonly used to classify women as underweight, normal weight, overweight, and obese prior to pregnancy. Prepregnancy BMI is a determinant of weight gain during pregnancy and birthweight (IOM, 2009).

  • Underweight is defined as BMI below 18.5 prior to pregnancy. The lower a woman's weight-for-height or BMI the more likely she is to be undernourished. Women who are underweight prior to pregnancy are at a higher risk for having a low birthweight infant, fetal growth problems, perinatal mortality and other pregnancy complications. (IOM, 1996)
  • Normal weight is defined as a BMI between 18.5 and 24.9.
  • Overweight is defined as a BMI greater than between 25.0 and 29.9. Being overweight prior to pregnancy is a risk factor for postpartum weight retention of prenatal weight gain. (IOM, 1996)
  • Obese is defined as a BMI greater than or equal to 30.0. Obese women are at greater risk of delivering a macrosomic infant and experiencing shoulder dystocia and other complications (IOM, 1996). Obese women are also more likely to develop gestational diabetes.

Prepregnancy Weight

BMI

Underweight

<18.5

Normal weight

18.5–24.9

Overweight

25.0–29.9

Obese

≥30

Maternal Weight Gain also called gestational weight gain refers to the amount of weight gained from conception to delivery. In 2009 the IOM published recommended weight gain amounts based on prepregnancy BMI for optimal infant health. Maternal weight gain is based on prepregnancy weight status and is considered to be a major determinant of birthweight as well as infant mortality and morbidity.

  • Ideal Weight is defined as a total weight gain within the range recommended by the IOM for each prepregnancy BMI classification. The ideal weight gain recommendations by IOM are considered as targets for identifying women who should be evaluated for inadequate or excessive gains (IOM, 2009). Gestational weight gain varies considerably among women of the same age, weights, heights, ethnic backgrounds and socioeconomic status. However, teenagers and black women continue to gain less than the recommended amount and are at a higher risk for poor outcomes (HP2010). A developmental health objective was established in Healthy People 2010 to increase the proportion of mothers who achieve the recommended amount of weight gain during their pregnancies.

Weight

Prepregnancy BMI

Total Weight Gain (lb)

Underweight

<18.5

28–40

Normal weight

18.5–24.9

25–35

Overweight

25.0–29.9

15–25

Obese

≥30

11–20

  • Less than (<) Ideal Weight Gain is defined as a total weight gain below the lower limits of that recommended by IOM for each prepregnancy BMI classification. Women with a low prepregnancy BMI and low gestational weight gain are more likely to have a low birthweight infant. During the second and third trimesters low maternal weight gain is a determinant of fetal growth, and is associated with smaller average birthweights and an increased risk of delivering an infant with fetal growth restriction. (IOM)

Prepregnancy Weight

< Ideal Weight Gain (lb)

Underweight

<28

Normal weight

<25

Overweight

<15

Obese

<11

  • Greater than (>) Ideal Weight Gain is defined as a total weight gain that exceeds the upper limit of that recommended by IOM for each prepregnancy BMI classification. High maternal weight gain has been recognized as a common nutritional problem in the U. S. with the prevalence being highest among low-income, black and Hispanic women. (IOM, 1996) Macrosomia, increased risk of cesarean deliveries and, possibly, spontaneous preterm delivery are all problems associated with very high gestational weight gain. In adolescents, high weight gain during pregnancy is association with neonatal complications. (IOM, 1996)

Prepregnancy Weight

> Ideal Weight Gain (lb)

Underweight

>40

Normal weight

>35

Overweight

>25

Obese

>20

 

Anemia during pregnancy is defined as less than the 5th percentile of the distribution of hemoglobin (Hb) or hematocrit (Hct). The distribution and cut off values are based on data obtained from clinical studies of European women who had taken iron supplements during pregnancy. (MMWR, 1998). The cut off values vary by trimester for pregnant women and are different from nonpregnant women. For nonpregnant women, anemia cut off values are established below the 5th percentile of the distribution of Hb or Hct from the third National Health and Nutrition Examination Survey for a healthy population. Trimester and age specific cut off values used in PNSS are shown below for pregnant and nonpregnant women, respectively. Because persons residing at higher altitudes have higher hematology levels, in PNSS Hb or Hct values are automatically adjusted for altitude.

Pregnancy Trimester

Hemoglobin

Hematocrit

First

11.0

33.0

Second

10.5

32.0

Third

11.0

33.0

Postpartum Age

Hemoglobin

Hematocrit

12 - < 15 yrs

11.8

35.7

15 - < 18 yrs

12.0

35.9

≥ 18 yrs

12.0

37.7

Pregnant women are at a higher risk for iron deficiency anemia because of the increased iron requirements of pregnancy. In pregnant women hemoglobin (Hb) or Hematocrit (Hct) levels drop during the first and second trimester because of blood volume expansion. Among pregnant women who do not take iron supplements Hb and Hct remain low during the third trimester. Longitudinal studies have shown that the highest prevalence of anemia during pregnancy is in the third trimester; therefore, the Healthy People 2010 objective monitors the prevalence of anemia during the third trimester of pregnancy. This objective seeks to reduce anemia in the third trimester among low income women from its baseline of 29 percent in 1996 to 20 percent in 2010. Pregnant women who have adequate iron intake have a gradual rise in Hb and Hct during the third trimester toward the prepregnancy levels (MMWR, 1998). Changes in the prevalence of anemia over time can be used to evaluate the effectiveness of programs designed to decrease the prevalence of iron deficiency. 

The analysis of postpartum anemia includes only records with valid Hb and Hct measurements taken at greater than 4 weeks or 28 days postpartum when Hb and Hct measurements are expected to return to prepregnancy or first trimester levels. After delivery, maternal hemoglobin is expected to increase as the expanded red cell mass of pregnancy contracts and iron returns to body stores.

Parity refers to the number of times a woman has been pregnant for 20 or more weeks regardless of whether the infant is dead or alive at birth (The current pregnancy is not included.). Parity, or the number of previous pregnancies, has been shown to impact the long-term health status of women and pregnancy outcomes, specifically birthweight, for some groups. A number of studies show that first-born children have a lower mean birthweight and are at greater risk of low birthweight than subsequent children (Kramer, 1987; Cogswell and Yip, 1995; Macleod and Kiely, 1988; IOM, 1985,). Multiparity at a young age (under 20 years) increases the risk of delivering a low birthweight baby (IOM 1996; Kramer 1987) and increased parity is associated with excessive maternal postpartum weight retention (Parker and Abrams, 1993) and with iron deficiency (Looker et. al. 1997).

Interpregnancy Interval is considered to be the amount of time between pregnancies and is calculated as the number of months between the date the last pregnancy ended and the date of the last menstrual period. Women with short interpregnancy intervals are at nutritional risk and more likely to experience adverse birth outcomes. Studies conducted by Lieberman and colleagues showed that women with an interpregnancy interval less than 18 months were at greater risk of delivering a full term small for gestational age (low birthweight) infant compared to women with interpregnancy intervals of 24 to 36 months. (IOM, 1996) Furthermore, interpregnancy interval of 3 months has been shown to result in an increase in the risk of delivery of a pre-term or small for gestational age infant as well as neonatal death. Shorter interpregnancy intervals also mean a shorter time for repletion of nutrient stores. (IOM, 1996)

Gestational diabetes refers to the presence of a type of diabetes that occurs during pregnancy (usually during the second or third trimester), if the body does not produce enough insulin to meet the extra needs of pregnancy. Gestational diabetes increases the risk of complications during pregnancy. Women with gestational diabetes are at risk of delivering macrosomic infants and developing type II diabetes later in life. (IOM, 1996) It is estimated that 2 to 3 percent of pregnant women will develop gestational diabetes. (IOM, 1996) A developmental health objective for 2010 has been established to decrease the proportion of pregnant women with gestational diabetes.

Hypertension refers to the presence of chronic hypertension or pregnancy induced hypertension. Hypertension is defined as an elevated arterial blood pressure. (NRC,1989) In adults hypertension is classified as a systolic pressure greater than 140 mm Hg and a diastolic pressure above 90 mm Hg. (WHO, 1978) Women with chronic hypertension prior to pregnancy are more likely to experience adverse pregnancy outcomes such as fetal growth restriction and abruptio placentae. (IOM, 1996) Pregnancy induced hypertension occurs in 5-9 percent of women and can lead to preeclampsia, eclampsia, and ultimately preterm delivery, fetal growth retardation, abruption placentae, and fetal death. (Zhang et. al. 1997)