During the first month of pregnancy, your baby-to-be grows at a rapid rate. But he's only about 1/5 of an inch long 4 weeks after conception, still not large enough to account for any weight gain by himself. Most women gain very little weight related to pregnancy itself during the first month. But mistakenly thinking you need to "eat for two" can definitely add pounds in the first few weeks of your pregnancy.
Typical Weight Gain in Early Pregnancy
The average woman gains less than 5 pounds in the first 3 months of pregnancy, and little of that occurs within the initial 4 weeks. Fluid retention, which occurs because of sodium retention related to increased progesterone levels, and increased breast size can account for a few pounds of early weight gain.
Overeating in Early Pregnancy
Although you'll want to eat a healthy diet, you do not need any additional calories in the first month of pregnancy for your baby's growth and development. However, many women increase their food intake, thinking it's for the baby's benefit. This can cause significant weight gain, even in the first month of pregnancy. If you start eating 450 extra calories per day -- the amount you'll need in the third trimester -- you can gain nearly 1 pound per week, or 3.6 pounds in the first month, from excess calorie consumption.