Your Baby

Your little one is about 5- 5 1/2 inches this week, measuring from crown to rump. As this week comes to a close, your baby will weigh close to 5 1/2 ounces or so. Your baby’s umbilical cord is lengthening and becoming thicker as it continues to transfer oxygen and nutrients to the placenta, which ultimately reaches your baby. The placenta, which already contains thousands of blood vessels, also continues its growth as the weeks go by; growing up to 3 cm thick by the time it’s delivered following your baby’s birth. Alveoli, which are tiny air sacs similar to balloons, are forming in your little one’s lungs. In the last several weeks of pregnancy, the cells in the alveoli produce a substance called surfactant, which helps the lungs expand after birth, so your baby can breathe. At birth, your baby will only have 10% of the millions of alveoli he or she will have as an adult.

Your baby’s reproductive system is continuing to develop. If you are having a girl, her ovaries now contain primitive egg cells; about six million (approximately six times the number she’ll have at the time of birth). The rest of her internal reproductive organs are completely formed now, as well. If you are having a boy, his testes have begun to descend from the abdomen and are apparent. The prostate glad is also developed in boys. If you have an ultrasound this week, it’s often possible to determine your baby’s gender, if you don’t want to wait until birth to find out. If you decide to wait and be surprised, make sure to tell the technician at the beginning of your ultrasound. Ultrasounds are typically fairly accurate depending on the skill level of the technician, position of your baby and also how far along you are, but definitely not 100%.

Your baby’s bones have been rather soft and rubbery up until now. His or her skeleton is gradually beginning to harden, transforming from soft cartilage to harder bone, although the bones will continue to remain somewhat flexible so your little one can fit more easily through the birth canal during delivery. This hardening process is called ossification and usually begins in the legs and inner ear. Practically all ossification is completed before your baby’s birth, except the fontanels of the skull, which are the two soft spots on your baby’s head. The fontanels allow the boney plates of your baby’s skull to flex during the birth process, so the head can be delivered more easily. These areas will harden completely by the time your baby is 1 1/2 to 2 years old.

Week 18, Your Body

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