Your Baby’s Development and Growth, Week-by-Week
Welcoming a new life into the world is an awe-inspiring experience for any parent, and from the moment you find out you’re pregnant, understanding your baby’s development becomes a top priority. Your baby will grow and change in incredible ways over the course of your pregnancy, transitioning from a tiny bundle of cells to a fully-formed human being. And after you give birth, you’ll continue to track your baby’s development milestones alongside your healthcare provider, oftentimes with the help of a growth chart. In this post, we’ll walk you through your baby’s development in each trimester, week by week. You’ll also learn what growth charts are, how to get one, and how you can tell if there is a problem with your baby’s growth. Fetal Development Before you even know you’re pregnant, the process of fetal development has begun. Your baby will go through three main stages of development between conception and birth: germinal, embryonic, and fetal. Stages of fetal development Germinal stage: This first stage of fetal development is also the shortest. It starts at conception, when the sperm fertilizes the egg in your fallopian tube, forming a zygote. Next, the zygote moves down your fallopian tube toward the uterus, dividing many times until it creates two separate structures (one will become the embryo, while the other will become the placenta). The tiny ball of cells that will later develop into the embryo is now inside a structure called the blastocyst. The blastocyst implants into the lining of the uterus, which triggers your body to start producing hormones to support a pregnancy. Embryonic stage: From weeks 3-8 of pregnancy, the blastocyst develops human characteristics and is now called an embryo. Your baby’s head, eyes, mouth, and limbs form during this time, along with the neural tube, which later will become the brain and spinal cord. By the end of this stage, most of your baby’s organs and life-sustaining systems will have started to develop, including their heartbeat. Fetal stage: Around week 9 of pregnancy, the embryo officially turns into a fetus. Thus begins the fetal stage, which lasts until birth and is when major fetal weight growth occurs. Your baby’s major organs (including their sex organs) grow and mature during this time, as well as fingernails, eyelashes, and hair. Stages of Baby Growth by Trimester You and your baby will experience big changes throughout each trimester. Read on to discover the nitty-gritty of your baby’s growth and development, week by week. First Trimester Weeks 1-2: Believe it or not, you’re actually not pregnant the first week or two of the time period that’s considered your pregnancy term. Conception usually occurs around two weeks after your last period begins, and your healthcare provider calculates your estimated due date by counting 40 weeks from the beginning of your last period. Week 3: This is when conception actually happens, through fertilization—sperm and egg come together in your fallopian tube to form a zygote (or multiple zygotes, if more than one egg is fertilized or the egg splits). Week 4: The tiny ball of cells, dividing rapidly and now referred to as a blastocyst, initiates the process of implantation by burrowing into the uterine lining. Week 5: The neural tube forms, which eventually will be the foundation for your baby’s brain and spinal cord. Week 6: Blood cells are starting to form, circulation begins, and your healthcare provider may be able to detect a heartbeat on an ultrasound. Small buds that will become your baby’s arms and legs form, as well as the structures that will eventually turn into their eyes, ears, and mouth. Week 7: Your baby’s brain and face are growing—their retinas start to form and depressions that will later become nostrils are now visible. Week 8: By the end of week 8, the embryo is now considered a fetus, and it’s about 0.5 to 1 inch long. Eyes become visible, ears start to form, and the fetus has webbed hands and feet. Week 9: Arms grow and elbows and toes appear. Your baby’s body shape starts to take on more of a human appearance, although its head is still 50% of its length. Week 10: No more webbing, as the arms, hands, fingers, feet, and toes have fully formed. Your baby can now bend their elbows, and their eyelids and external ears continue to form. Week 11: Your baby’s external genitals start forming, and buds where teeth will develop appear. While it’s too soon to feel any kicks, your baby is starting to explore movement through things like opening and closing their fists and mouth. Week 12: Your baby is growing fingernails, and at this point, all their organs, limbs, bones, and muscles have started developing. By the end of the first trimester, the fetus is roughly 2.5 to 3 inches long. Second Trimester Week 13: Your baby’s vocal cords form and their head starts to become more proportionate to the rest of their body. Some of your baby’s bones start to harden, particularly in the skull and long bones. Week 14: External genitals fully develop this week, making your baby’s sex more apparent. Their skin begins to thicken and fine hair starts growing. Week 15: Lungs start to develop, and some organs, such as the intestines and ears, move to their permanent position. Your baby is starting to smile! Week 16: While their movements are too slight to be detected by you at this point, your baby’s limb movements are becoming coordinated and can be detected via ultrasound. At this point the fetus is roughly 5 inches long and weighs around 4 ounces. Week 17: Your baby starts to put on fat and their toenails develop. Week 18: Your baby’s ears begin to stand out on the side of their head and they may start to hear sounds. At this stage, your baby’s digestive system has started working. Week 19: Your baby can hiccup! They are getting stronger, and most moms start to feel them kicking. Plus, a