Unlike the equation used to calculate your BMI, the process of putting on pounds is not an exact science. Your rate of weight gain will depend on a variety of factors, such as your metabolism, your activity level and your genetics — just another reason why it’s important to keep up your doctor appointments throughout your pregnancy.

These vary by trimester:

During your first trimester, your baby is still tiny, which means you don’t need to gain more than a total of three to four pounds. However if you’re suffering from morning sickness, you might not gain an ounce (or might even lose a little). That’s OK, as long as your appetite picks up and you make up for those pounds in the second trimester.
In your second trimester, your baby starts to grow in earnest. Your weight gain should pick up so that, if you started out pregnancy at a normal weight, you’d gain a total of about 14 pounds.
In your third trimester, baby’s weight gain will pick up steam, but yours may start to taper off for a net gain of about 10 pounds. Some women find their weight holds steady or even drops a pound or two during the ninth month, when ever-tighter abdominal quarters can make finding room for food a struggle.

How closely will you be able to follow this formula? Realistically, not that closely. There will be weeks when your self-control will waver and it’ll be a rocky road (by the half gallon) to your weight gain total. And there will be weeks when eating will seem too much of an effort. Try not to stress over the scale. As long as your overall gain is on target and your rate averages out to the above, you’re right on track.

Ever wonder how your pregnancy weight is distributed in your body? It may feel like it’s all in your belly, but that’s not actually the case. For a 30-pound weight gain, here’s the average breakdown:

Baby: 7.5 pounds
Placenta: 1.5 pounds
Amniotic fluid: 2 pounds
Uterine enlargement: 2 pounds
Maternal breast tissue: 2 pounds
Maternal blood volume: 4 pounds
Fluids in maternal tissue: 4 pounds
Maternal fat stores: 7 pounds
You need weight in all these areas in order to have a healthy baby and to get your body ready for motherhood (including preparing your body for nursing).

For best weight gain results, keep your eye on the scale, since what you don’t know can throw your weight way off-target. Weigh yourself:

At the same time of day
Wearing the same amount of clothes (or none at all)
On the same scale
Once a week (more often and you’ll drive yourself crazy with day-to-day fluid fluctuations; if your scale-phobic, twice a month should do the trick)