No one knows how big your baby is. And no one else can tell you what your baby weighs either, because babies have yet to master the art of standing on a scale.
Luckily, there are plenty of other ways to track your growing little bundle’s size during pregnancy! Doctors use many methods to predict fetal weight and monitor growth throughout gestation. Using ultrasounds, they can calculate the size of different body parts like your baby’s head and abdomen, as well as measure overall growth. This information helps to determine how much weight gain you should have during pregnancy.
The Centers for Disease Control (CDC) say that your baby is supposed to grow about two inches each month during the first trimester and about an inch a month during the remaining months.
When looking at a fetal growth chart by week, you may wonder why the age is not just measured from conception. The reason for this is simple: scientists can’t use a time-lapse video of a human embryo developing to plot the measurements because not all embryos develop at the same speed. It would be impossible to compare where exactly 28 days fall for one growing embryo versus another.
Every baby develops differently, meaning that the week of pregnancy you are in can make a large difference in how your fetus is developing. It’s standard to measure age from the first day of the mother’s last normal menstrual period (LMP), which typically corresponds with the day after conception, but this doesn’t always work out.
While some fetuses may be the same size as others at a given age, this is not always true. There are a variety of factors that can contribute to a small or large baby, including genetics and lifestyle factors such as diet and exercise.
The fetal growth charts used for this study were developed at Parkland Hospital and are based on ultrasonic measurements of the fetus during the routine second trimester examination. The height and weight for each week, as a percent of a standard population, is determined by the ratio of the mean for that week to the mean of the standard population. The growth rate, in terms of standard deviation scores, is determined by dividing the difference between this ratio and 1.0 into thirds. This assigns a weight of +1 for normal values > 100%, 0 for measurements within 10% of 100%, and -1 for values <10% of 100%. The mean and standard deviation scores are then plotted on the chart.
