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Your Baby’s Development in the Womb

Pregnant woman with ultrasonic baby shot isolated on white

Pregnancy can be an exciting time and Rocky Mountain Women’s Health Centers consider it a privilege to care for you and your developing baby.  Here are some fun facts about a baby’s development in the womb.

The First Trimester (conception through week 12)

Pregnancy begins when a sperm fertilizes an egg. Because we often don’t know exactly when conception takes place, doctors calculate the timetable for your pregnancy based on the first day of your last menstrual cycle. Your due date is estimated to be at 40 weeks.

Dramatic change occurs in the embryo during the 5th to the 10th week of pregnancy. Here’s what happens during this stage:

  • All of the baby’s major parts are developed, including: nervous system, heart, face, arms and legs, sexual organs, muscles.
  • The embryo’s cells begin to multiply and change into hundreds of different types of cells needed to make up the human body
  • The placenta develops which provides oxygen, water and air to the embryo through the umbilical cord
    The amniotic sac develops which surrounds and protects the developing baby
  • The final two weeks of the first trimester are referred to as the fetal state. During this period of time the fetus gains weigh, and organs and body parts continue to develop. By the end of the 12th week, the fetus is usually about 3 inches in length.

The Second Trimester (weeks 13 through 26)

The second trimester occurs during week 13 through 26 of your pregnancy. This is a period of continued growth and development for the baby. During this period, the baby’s head is the biggest part of its body, but the rest of the body will get longer and by the end of the second trimester your baby may be 9 inches or longer.

Around the 20th week you may begin to feel your baby move. These movements help develop muscles as your baby kicks, such and opens and closes its hands. A few weeks into the second trimester, an ultrasound may be able to confirm the baby’s sex. Other highlights include:

  • The baby begins to hear sounds
  • The baby’s eyes may open around the 20th week
  • The baby develops fingerprints and footprints
  • Fine hair begins to appear

The Third Trimester (week 27 to the end of pregnancy)

The third trimester runs from the week 27 to the end of the pregnancy. At this point the baby’s organs and body parts are developed and working. The third trimester is a time for everything to continue to grow and mature. The lungs are the last major organ to complete their development.

Other highlights from the third trimester include:

  • Kicks, punches and moving often occur in the first weeks of this trimester
  • As the baby gets larger with less room in the uterus, there may be fewer kicks, replaced by stretches and rolls
  • Eyelashes and eyebrows develop and sometimes a full head of hair
  • Most babies move to a head-down position near the end of the trimester in preparation for birth
  • The majority of babies weigh between 6 and 9 pounds at birth and measure 19 to 21 inches long

And, did you know that only 5% of babies are born on their actual “due date”?

Breastfeeding still optimal for infant health

Many in the medical community are concerned about the declining number of mothers who are exclusively breastfeeding. There are a number of benefits for both mom and baby that result from breastfeeding. In most circumstances, breastfeeding is considered the best nutrition for a newborn. There are components of breast milk that are not offered in formula that make breast milk the first choice.

The World Health Organization and the American College of Obstetricians and Gynecologists (ACOG) both recommend exclusive breastfeeding of infants until six months, then continuing to breastfeed for as long as possible. The CDC stated in its Healthy People 2010 Report that, “Breast milk is widely acknowledged as the most complete form of nutrition for infants, with a range of benefits for infants’ health, growth, immunity and development”.  Breast milk contains antibodies (disease-fighting cells) that protect infants from germs and illnesses. It is also easier to digest for most babies. Studies indicate that nursing babies have a lower risk of ear infections, diarrhea, stomach viruses, respiratory infections, asthma, obesity, diabetes, childhood leukemia and atopic dermatitis (dry and itchy skin).

Despite the recommendation to breastfeed, there are some circumstances where breastfeeding is not an option due to certain medications or medical conditions. The baby may also not get the nutrition it requires and supplementation with formula is necessary. There are good options for formula and moms can use formula confidently if needed to supplement.

Breastfeeding is a unique experience for every woman and can vary even with each child. If a woman is struggling with her breastfeeding, she needs to feel supported in expressing her concerns. Breastfeeding comes easily for some and is very difficult for others. There are many resources available to support breastfeeding moms. If you have questions or concerns, talk to your provider.


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