Pregnancy Stages of Pregnancy Week by Week Pregnancy Week 3 Find out how your baby is developing in week three and learn the early signs of pregnancy, along with the importance of choosing the right doctor. By Parents Editors Parents Editors The Parents Editors byline means there is an experienced group of editors and journalists behind the scenes of the article. Learn more about the Parents team, our core values, and editorial guidelines. our editorial guidelines Updated on May 17, 2022 Share Tweet Pin Email Your due date will be calculated from the first day of your last menstrual period. Conception occurs about two weeks from this day, and that's when you're truly considered pregnant. In just 40 short weeks, your baby will grow from the size of a tiny seed to the size of a plump watermelon. Baby's Length: 0.0in.Baby's Weight: 0.0oz. Mom's Changing Body You may experience some light vaginal bleeding when your egg implants, which many women mistake for a period. In fact, spotting that's lighter than your usual menstrual flow is one of the early signs of pregnancy. Do give your doctor a call, however, if the bleeding is fairly heavy or you experience abdominal pain, like a sharp stabbing in your pelvis or even mild cramping on just one side. Although uncommon, these symptoms could indicate an ectopic, or tubal, pregnancy. This happens when the fertilized egg attaches itself somewhere other than the uterine wall—usually in a fallopian tube. The most common causes of an ectopic pregnancy are inflammation of the fallopian tubes and an infection in the uterus, fallopian tubes, or ovaries. SEBASTIAN KAULITZKI/SCIENCE PHOTO LIBRARY Baby's Development Amazingly, your child's sex and all of her inherited genetic characteristics—such as eye color, hair color, skin, and body type—have been set since the moment of conception. Your developing baby, now called a zygote, has 46 chromosomes—23 from you and 23 from your partner. These chromosomes help determine your baby's sex and traits such as eye and hair color, and, to some extent, personality and intelligence. After fertilization, the ball of cells, now an embryo, will wrap up its journey through the fallopian tube and burrow itself into the wall of your uterus for nourishment—a process known as implantation. Read More Weekly Tip Get yourself a good doctor—pronto! Having a healthcare provider you like and trust is key. After all, this is the person who will guide you through your entire pregnancy, labor, and delivery, along with the millions of questions, tests, and emotional ups and downs along the way. If you don't currently have an ob-gyn you love, ask friends, family, and coworkers for recommendations or research practices until you find someone you feel comfortable with. And, hey, your choices are not limited to regular obstetricians. You might also want to investigate alternative or additional types of care—including midwives or family physicians. Shutterstock Week 3 Ultrasound This week is when your pregnancy really begins. At some point, the sperm joins with the egg as it makes its way from the ovary through the Fallopian tube and then into the uterus. Fertilization takes place inside the Fallopian tube. Once together, the cells begin to divide rapidly so that next week, a sonographer may be able to capture baby-to-be's beginnings during an ultrasound examination. Read More Watch Baby's Growth This Week's To-Do List Look for the early signs of pregnancy.Read More Buy an at-home pregnancy test.Read More Learn what foods you should avoid during pregnancy.Read More This Week's FAQs How Accurate are Home Pregnancy Tests? Must-Reads 14 Very Early Signs of Pregnancy Implantation Symptoms: Can You Experience Signs of Conception? 9 Things You Didn't Know About Sperm Your Chances of Getting Pregnant Every Day of the Month Was this page helpful? Thanks for your feedback! Tell us why! Other Submit