How much weight you should gain during each trimester of pregnancy, according to doctors

• During pregnancy, you should typically gain 2-4 pounds during the first trimester.
• After that, you should gain about a pound a week - more rapid weight gain could put you at risk.
• You should consume the most amount of calories (2,400 calories/day) in your third trimester.

Eating for two might sound like a nice perk of pregnancy, but unfortunately, that's not really how it works. Here's how to gain weight safely during each trimester of pregnancy.

Weight gain is relative

How much weight you gain while pregnant is relative to your size and shape before you were pregnant, says Alexandra Stockwell, MD, who has spent over a decade in family medicine.

"In general, it's expected that a woman gains 2 to 4 pounds in the first trimester and then 1 pound per week thereafter," Stockwell says. "Typical weight gain is 25 to 35 pounds, but it's healthy for women who are underweight to gain 25 to 40 pounds and women who are overweight to gain 15 to 25 pounds."

For someone carrying twins, the amount of weight you gain shouldn't double, but there will be a marked increase - about 37 to 54 pounds if you began your pregnancy at a healthy weight, based on body-mass index (BMI).

At birth, a baby averages about 7 pounds. So if the average woman should gain 25 to 35 pounds, then where does the rest of the weight come from?

"It's due to larger breasts, expanded uterus, the placenta, and amniotic fluid," Stockwell says. "Increased blood volume, fluid volume, and fat stores account for the rest."

First trimester: You won't gain much

During the first trimester, you can expect to gain 2 to 4 pounds.

The baby is only about 5 centimeters long, so most of the weight is from increased fat stores, the uterus, and placenta, which are all preparing for when the baby starts to grow larger over the next two trimesters.

Second trimester: You'll gain about one pound per week

From the second trimester through the end, you should expect to gain about 1 pound per week.

Part of that is because in the second trimester you'll slowly start increasing your caloric intake by about 340 extra calories a day, according to the Centers for Disease Control and Prevention. But this number varies.

For example, England's Royal College of Obstetricians and Gynaecologists recommend that women increase their caloric intake only by 200 calories in the third trimester.

Therefore, consult your obstetrician about how many additional calories are right for you and when.

Third trimester: You'll eat around 2,400 calories per day

By the final trimester, US doctors recommend that add another 110 additional calories to your diet.

"If a woman's normal caloric intake is 1,800 per day, that's a good amount to stick with during the first trimester," Stockwell says. "By the third trimester she should be consuming 2400 calories per day."

By the third trimester, the baby will be gaining about half a pound a week. This is when it's especially important to get enough calories and nutrients in your diet. If you're not eating enough to provide your baby with the food it needs to fully develop, the baby is going to get that nutrition some other way.

"One very important reason for pregnant women to eat well is so that they stay healthy and do not become nutritionally depleted by providing for the growth of the baby," Stockwell says. "For example, if mom doesn't have enough calcium in her diet, the baby's bones will form from calcium taken from mom's bones and transferred through the bloodstream to the placenta where it is used by the baby."

But these extra calories can't just come from anywhere, says dietitian Sally Poon.

"Many often women experience sugar or carbohydrate cravings during pregnancy and overeat cakes, biscuits, or candies," Poon says. "This snacking behavior can lead to rapid weight gain and thus increase the risk of gestational diabetes."

When does weight gain become a risk?

If you find that you're gaining too much weight, too quickly - more than a pound a week - it's time to see a doctor or nurse who will monitor your weight.

In addition to gestational diabetes, excess weight gain during pregnancy can lead to other health problems for the mother, like fetal macrosomia or difficult delivery. Moreover, it makes that pesky pregnancy weight harder to lose after birth if there's more of it.

Further, rapid weight gain - more than 2 pounds a week - can be a sign of preeclampsia, and you should speak with your doctor immediately.

On the flip side, not gaining enough weight can be problematic, too, which is why dieting and weight loss during pregnancy are not recommended during pregnancy.

"It's bad for the pregnancy and baby if you are malnourished," Stockwell says. "This correlates with inadequate weight gain, but it's the malnourishment which is the issue and not so much the lack of weight gain."

Insider's takeaway

Though you may not gain much weight in the first trimester, you should expect to gain about a pound a week after that. In the second trimester, your caloric intake should go up by about 340 calories a day, and you should add about 200 calories on top of that during the third trimester.

Ultimately, Stockwell encourages expectant mothers to not get hung up on the numbers during pregnancy.

"What's most important is that the food is nutrient-dense, meaning it contains vitamins, minerals, and all the building blocks needed to form a human body, like calcium for the bones, and B12 or folic acid to prevent spina bifida."

50 essentials to bring to the hospital when giving birth: A printable guideVaginal discharge may change during pregnancy - here's what to expect and when to be concerned13 early signs you may be pregnant7 questions your OB-GYN wishes you would ask them when pregnant