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 and be about the size of a bell pepper as opposed to the size of a pear in week 17 of pregnancy.
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.
Your Body in Pregnancy
Your body has likely expanded enough for others to realize you’re pregnant and not just gaining a few extra pounds. Your uterus is similar to the size of a small melon and can be felt just below your belly button. Don’t be surprised if your “inny” becomes an “outy” soon! You are probably gaining weight quite steadily at this point; possibly up 10- 13 pounds by now.
You’re looking pregnant, but it’s still probably pretty easy to get your shoes and socks on without too much trouble, although your pre-pregnancy pants are too tight to zip and your shirts may be starting to become snug this week. If you haven’t already; it’s time for maternity clothes shopping. There are so many stylish maternity clothes options out there, you should have fun shopping for your new wardrobe! Also, don’t be surprised if some of your girlfriends who have children offer to give you (or let you borrow) some of their maternity clothes. This is a great way to get a maternity wardrobe without spending a lot of money.
You may be scheduled for an ultrasound exam this week or within the next couple of weeks. Ultrasounds are routinely performed sometime between 18 and 20 weeks along as part of your prenatal care. You may be asked to drink a certain amount of water prior to your ultrasound, which will help produce a better picture, because a full bladder helps lift the uterus up so it can be seen more easily. Your ultrasound will probably last anywhere from 15- 25 minutes on average.
An ultrasound done at this time can detect many birth defects and structural abnormalities, determine the position of your baby and the placenta, check amniotic fluid level and calculate gestational age. Body measurements are taken to estimate your baby’s age and also to ensure your baby is growing properly. Estimates on age or weight aren’t all that accurate during this time and into the third trimester. The overall skill of the technician and also your baby’s position helps, but the earlier in pregnancy the ultrasound is done, the more accurate it is for estimating the age.
Right after conception (during the first trimester), babies grow about the same speed, but as time goes on, babies tend to start growing at different rates. During the first trimester, estimates using ultrasound are usually within a week of the correct age. By the time the second trimester rolls around, they can be off by two weeks or more.
Some measurements which will likely be taken include the distance around your little one’s abdomen (abdominal circumference), the distance across your baby’s head, the distance around the outside of your baby’s head (head circumference) and the length of your baby’s upper leg bones (femur length). Body parts will be examined including your baby’s brain, spine, kidneys, stomach, bladder and the four chambers of your baby’s heart. The umbilical cord will be examined as well. If any birth defects or problems are suspected, you’ll be scheduled to have a more detailed ultrasound done (often called a “level II ultrasound”