Nine months can seem a long time as your body is going through all the changes that pregnancy brings, but it may seem relatively short when you consider the dramatic changes and growth your baby goes through. Considering that a fetus begins as small as the size of a period on a printed page and develops into a 7 lb. baby that can sustain life outside the womb—all within 240 days—a baby's gestation may be one of nature's greatest feats.
Your egg and your partner's sperm unite in one of two fallopian tubes to form a one-celled zygote, which contains 23 chromosomes from you and 23 from your partner.These chromosomes determine your baby's gender, her hair and eye color, and can influence even her personality and intelligence, reports MayoClinic.com.
First month: Your now-fertilized egg moves through the fallopian tube and implants itself in the lining of your uterus. Some of the cells of the zygote will multiply and become the placenta, and others will form into your baby. In these first four weeks, arms and legs start to develop as well as the brain and spinal cord. The heart begins beating by the fourth week. Your embryo is now 1/2-inch long and weighs less than 1 oz.
Months Two and Three
According to the American Congress of Obstetricians and Gynecologists, or ACOG, the second and third months show rapid change. Weeks five through eight see the eyelids form, though they remain closed, and development of the inner ear. Bones, ankles, wrists, fingers, toes and genitals begin to develop as well. ACOG also states that by week eight, all of your baby's major organs and systems, such as nervous, cardiac and respiratory, are well underway.
Month number three marks the buds of his teeth, formation of soft nails and the intestines. Your baby's back bone can flex, and his hands and fingers are distinguishable. He is now 1 inch long and still weighs less than 1 oz.
Months Four and Five
Month four will see arms and legs flexing, the formation of external sex organs and development of eyebrows, eyelashes and fingernails. The placenta will be completely operational, furnishing nutrients and oxygen to your baby through the umbilical cord. Your baby can swallow, and her kidneys will begin producing urine. She is now 6 to 7 inches long and weighs about 5 oz.
The fifth month comes and you're more than half way through your pregnancy. Your baby is quite active now and you will most likely feel your first flutters and kicks. She can suck her thumb and regular sleep/wake cycles have begun. All of the eggs she will ever have in her ovaries are present. She is 10 inches long and weighs between 1/2 and 1 lb.
Months Six and Seven
The sixth month brings real hair growth and opening of the eyelids. Your baby's brain develops rapidly this month and lungs are fully formed. Finger and toe prints are individualized. The length is now about 12 inches and weight is 1 to 1 1/2 lbs.
Month seven brings sensitivity to light, full stretching and grasping/releasing of the hands. Your baby also begins to respond to sounds. He is 14 inches in length and weighs 2 to 2 1/2 lbs.
Months Eight and Nine
Month eight: The major development of all organs and body systems is complete. Your baby's job now is to put on weight and flex her muscles. The bones will harden, though the skull will remain somewhat soft so that she may pass through the birth canal. Her taste buds are developed and she can distinguish sweet from sour. Don't be surprised when you begin to feel her hiccuping. Babies now average 18 inches and 5 lbs.
Month nine, weeks 36 to 40, are the home stretch. The lungs mature in these last four weeks and are ready to function, and most babies will turn their heads downward for exit from the womb. As fat forms beneath it, skin smooths out and sleep patterns develop. Most babies gain 1/2 lb. per week and the average length stretches to 20 inches. Birth weight will most likely be between 6 and 9 lbs.