If you are currently pregnant, you probably know that your midwives and doctors are pretty obsessed with your weight gain—we check it at every visit. The recommended total weight gain is between 28 and 40 pounds, depending on how much a woman weighed before getting pregnant.

The reason we check in so frequently is that healthy weight gain can decrease the risks of pregnancy and birth complications, and some longer term health issues for the baby. It can also play a role in how big a baby is when he is born.

The second and third trimesters of pregnancy are generally when women experience the most weight gain. The American Pregnancy Association tells us that women can expect to gain 1 to 4.5 pounds during their first trimester, and then 1 to 2 pounds per week during their second and third trimesters.

While you may be the most aware of the extra pounds towards the end of your pregnancy, it turns out that the weight we gain in the beginning of our pregnancies may have the most impact on how big our babies are at birth.

A recent study of 1,164 women looked at pre-pregnancy weight, and then weight gain during 10 specific intervals during pregnancy. Researchers found that while pre-pregnancy weight impacted the babies’ birth weight, the only pregnancy weight gain that seemed to affect the birth weight happened in the first 18 weeks.

During week 1 through 14, for every pound a woman gained, her baby’s weight increased by 13.6 grams (0.48 ounces). And for every pound she gain between week 14 and 18, the baby’s birth weight increased by 26.1 grams (0.92 ounces).

I have to say, I find these findings comforting. So many women, in my experience, get really stressed by their weight gain during the second half of pregnancy—we now have reason to believe that they don’t have to stress.

Certainly, women should still eat healthy, exercise and aim to keep their weight gain within the recommended range. And, women that are thinking about becoming pregnant might consider making sure they are as healthy as possible before trying to conceive.

But, for all those mamas who sweat every pound towards the end, you can breathe a sigh of relief. Just do the best you can mama—your best is pretty amazing.