Pregnancy is an amazing and exciting time for new parents. As mom carries on with her daily life, a few cells inside her grow into a living, breathing, kicking, screaming baby, in just 40 short weeks. While mom's uterus grows from the size of a baseball to a watermelon, her baby is growing longer, gaining weight and developing inside and out.
Length
During the first trimester, according to Elizabeth Stein from Parents magazine, the fetus will grow about 2.5 inches, from the size of a grain of rice at 4 weeks to the size of an apple by 12 weeks. By the end of the second trimester, it will be between 11 and 14 inches long, with a growth “spurt” in the fifth month of 2 to 6 inches. By their due date, most babies are 19 to 21 inches long.
Weight
Stein writes that, by the end of the first trimester, a baby will weigh a little more than 1 ounce. During the second trimester, the fetus will gain at least 1 pound. The baby will gain most of his weight in the final trimester as fat deposits begin to form under the skin: 2 pounds in month 7, 2 more in month 8 and up to half a pound a week after that.
Physical Development
According to Heidi Murkoff, Arlene Eisenberg and Sandee Hathaway, authors of “What to Expect When You’re Expecting,” a fetus grows from a shapeless mass of cells into a tiny, tadpole-like embryo by 8 weeks. Stein writes that the baby’s arms and legs begin to grow in the first month. Ears, ankles, wrists, fingers and toes develop in the second month. External genitalia develop around the third month, according to Murkoff et al. During the fifth month, ears are well developed and able to hear sound outside the womb. By the sixth month, the baby’s skin is covered with fine, soft hair, and his eyes begin to open.
Internal Development
A baby’s heart and lungs begin to form in the first month, and his heart begins to beat by the 25th day, according to Stein. The neural tube, which becomes the brain and spinal cord, also begins to form in the first month. All major organs and systems form within the first two months and mature throughout the rest of the pregnancy. According to Murkoff et al, the baby’s skeleton gets harder in the fourth and fifth months. Brain development becomes complete in the third trimester, and his lungs are mature by 37 weeks.
Movement
According to Murkoff et al, a fetus starts moving its arms and legs around the fourth month, the same time when he begins to suck his thumb and make breathing movements. During the fifth month, the muscles strengthen and nerve networks expand, making the fetus much more active. By the final months, the baby has grown to take up most of the uterus, and kicking room pretty much runs out, but most babies settle into a head-down position near the birth canal in the ninth month.
References
- Parents: Trimester by Trimester
- "What To Expect When You're Expecting;" Heidi Murkoff, Arlene Eisenberg, Sandee Hathaway; 2002
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