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The Size of an Unborn Baby at One Month

author image Norma DeVault
Norma DeVault, a registered dietitian, has been writing health-related articles since 2006. Her articles have appeared in the "Journal of the American Dietetic Association.” She holds a Doctor of Philosophy in human environmental sciences from Oklahoma State University and a Master of Business Administration from the University of Tulsa.
The Size of an Unborn Baby at One Month
Pregnancy timelines help a mother visualize her baby at each stage of development.

From the time a woman learns she is pregnant, with both anxiety and exhilaration, she wonders how her baby is growing and developing and what to expect next. A pregnancy time line helps her keep track of the many changes taking place within her womb and in her own body. Every pregnancy follows the same series of milestones in the same order, but knowing the size and stage of development of the infant can be very reassuring to a new mother.

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To avoid confusion, pay particular attention to whether a pregnancy time line reflects weeks of pregnancy or weeks of gestation after conception. Most medical sources base the pregnancy time line from the beginning of the menstrual cycle two weeks prior to conception. That means there is a difference of two weeks between time lines that refer to weeks of pregnancy and time lines that refer to weeks of gestation, according to


The developing infant one month into the pregnancy, which is two weeks after conception has just completed the germinal period as a zygote, or fertilized egg, and is entering the embryonic period of development, according to John Santrock in “Life-Span Development.” Growth in the embryonic stage is very rapid. The embryo doubles the number of cells in its body every 24 hours.


Within the first month after conception, the fertilized egg has attached to the lining of the uterus. Differentiation between cells that will grow into a baby and those that will form the placenta has occurred. Arms and legs as well as the brain and spinal cord have begun to form. Facial features are beginning to form and the neural tube along the baby’s back is closing. The heart and lungs have begun to develop. After one month of gestation, which is six weeks into the pregnancy, the heart begins to beat and pump blood, according to

Size at One Month

One month into the pregnancy, or two weeks after conception, the fertilized egg becomes attached to the uterus. By four weeks after conception, the embryo is 1/2-inch in length and weighs less than 1 oz., according to the American Congress of Obstetricians and Gynecologists. After one month of gestation, at six weeks into the pregnancy, the embryo is still less than 1-in. long and weighs less than 1 oz.

Critical Period

Times of intense development and rapid cell division are critical, according to Eleanor Whitney and Sharon Rolfes in “Understanding Nutrition.” During the fourth week of gestation, the central nervous system, heart, ears, eyes, legs and arms are in their critical period of development. This time is critical in the sense that each organ and tissue developing at this time is most vulnerable to adverse influence such as nutrient deficiency, nutrient excess, toxins or pathogens. For instance, the critical period for neural tube development is from 17 to 30 days of gestation when many women do not yet know they are pregnant. The neural tube is the beginning of the brain and spinal cord. Failure of the neural tube to close properly at this time results in central nervous system disorders. In the United States, 1 in 1000 newborns has a neural tube defect, according to Whitney and Rolfes. Supplements of the vitamin folate before and during the first trimester can reduce the risk of neural tube defects.

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