Welcome to the first trimester of pregnancy. This is the start of a wonderful journey, and it’s natural to have lots of questions about what’s in store for you in the weeks and months ahead. We’ve compiled some essential information on fetal development during the first trimester, common early pregnancy symptoms, and what’s coming your way this trimester. Show How Long Is the First Trimester?The first trimester is about 13 weeks long, and it actually starts before you become pregnant. This is because your estimated due date is usually calculated from the first day of your last menstrual period (LMP). A full-term pregnancy is roughly 40 weeks long, so your healthcare provider will calculate 40 weeks from the start of your last period to estimate when you’re due. If you haven't been to your provider yet, you can use our Due Date Calculator to get a quick estimate. For more on this, read about how far along you are. Keep in mind that most babies aren’t born exactly on their due date, which are only estimates. Rather, they are typically born in the two week period before or after the date. Your pregnancy is considered full term pregnancy at the start of 39 weeks. The trimesters of pregnancy are composed of 40 weeks. Take a closer look at what happens during each individual week of pregnancy. The weeks of pregnancy are sometimes broken down into months, but there’s some variance in how the weeks are grouped into months. This is because 40 weeks don’t divide neatly into 9 months, and because months typically are a little longer than 4 weeks. Your Baby’s Development in the First TrimesterDuring the first trimester of pregnancy, what starts as a tiny bundle of cells quickly turns into an embryo and then a fetus that's about the size of a large plum. In these first few weeks of pregnancy, your little one’s brain, spinal cord, heart, and tiny limbs — complete with fingers and toes — will form. Meanwhile, your uterus is becoming a comfortable home for your little one, who will be nourished by the developing placenta and umbilical cord. By the end of third month of pregnancy, all essential organs and body parts will be in place — albeit in teeny-tiny size. There’s a lot happening in the first trimester. Here are a few of the most exciting fetal development milestones: 4 Weeks: ImplantationThe ball of rapidly dividing cells, called a blastocyst, implants in the uterus. This is when some moms-to-be experience implantation bleeding. The inner cells will become the embryo and the outer cells will become the placenta. The placenta will provide nourishment to your baby from now until delivery. 6 Weeks: Taking ShapeAround six and seven weeks, the heart, lungs, and other key organs start to develop, and the head and limbs also take shape. What was a ball of cells just weeks ago is forming a more recognizable c-shape. 9 Weeks: In MotionFrom around 9 weeks, your little one, now known as a fetus, may start moving around. However, you probably won't be able to feel any movement until the second trimester. For more on this topic, read up on quickening and fetal movement. 10 Weeks: Fingers and ToesThis week your baby's fingers and toes lose their webbing and continue to grow longer. It won't be long until you're able to count them! 10 or 11 Weeks: The Sound of Your Baby’s HeartFrom around 10 or 11 weeks, your little one’s heart may be beating loudly enough to be heard with a Doppler ultrasound, which your healthcare provider may perform at your next appointment. Illustration of Fetal Development Week by WeekClick through the illustrations below for a visual representation of how your little one develops week to week during the first trimester: Here are just some of the things you may be doing or that may be coming your way in the first trimester:
First Trimester SymptomsAlthough every pregnancy is unique, here are some of the most common symptoms of the first trimester:
FAQs at a Glance
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