This chart has been compiled using the median, or most common, numbers for each week during pregnancy. But of course just like everyone else, babies come in all shapes and sizes. You've no doubt heard about babies weighing fourteen pounds at delivery or five pounds when born. Your doctor will use ultrasound to monitor the development of your baby during pregnancy to make sure that it is healthy and normal.
As your baby grows its legs will lengthen and start to uncurl. From week twenty on your doctor will measure your baby from head to toe, rather than from head to bottom. You will notice a growth spurt in the chart that marks this change in your baby's development.
You can download a full-sized copy of this chart along with a table of the median numbers (we've included both PDF and Excel spreadsheet versions). You may choose to plot the size of your baby on the chart to see how it compares to the national average, or enter them into our spreadsheet for comparison.
Most babies are about eight pounds (3.7 kg) and twenty inches (51 cm) when delivered at term. During the last four weeks of pregnancy babies may gain half a pound each week, so babies born preterm are generally lighter, although they are much closer to the average size. A postterm baby may continue to grow at the same rate if the placenta continues to function properly. If the placenta has started to break down the baby may not grow much larger until after it has been delivered.
Do genetics or uterine environment determine a baby's growth?
In fact both genetics and environment are responsible for the delivery size and weight of a baby. Obviously the parent's genes are key factors in determining whether a baby will be small or large, but they are tempered by a number of additional factors. For instance, a mother whose diet lacks the necessary nutrition, or who smokes will have a baby that is smaller than expected at delivery. Similarly, a mother who has unrecognized or uncontrolled diabetes during pregnancy will deliver a baby that is larger than normal. Some doctors estimate that estimates of fetal size are only about 50% accurate, and that even with ultrasound, there is a lot of guesswork used when determining the size of a baby.
Fetal development during the first trimester
During the first trimester most of your baby's structure is formed. The internal organs, face and body structures are developed and prepared for the growth that takes place during the second trimester.
The vertical red line in the chart shows the point of conception (week two of your pregnancy). Most early home pregnancy tests can confirm pregnancy about 10 - 14 days after conception, marked by the vertical blue line. This line also marks the point where important internal structures such as the central nervous system and genitourinary system, as well as facial development begins: making a compelling argument for following a healthy diet and quitting smoking as soon as you know you are pregnant.
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