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.

Week by week baby size image. Week by week baby size image.

How big is baby?

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.

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Fetus at Week 3 What the baby looks like at week 3 of fetal development. Med ART Studios Explore More

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. If you're having multiples, the deed has already been done. Fraternal twins occur when two separate eggs are fertilized by two different sperm and each baby has his own placenta and amniotic sac. If one fertilized egg splits and develops into two fetuses, the result is identical twins. They may share a placenta, but each baby usually has a separate amniotic sac.

Baby Week 3

The blastocyst moves down the fallopian tube towards the uterus. If the blastocyst gets lodged in the fallopian tube before reaching the uterus, it is referred to as an ectopic pregnancy.Conception occurs upon the union of a sperm and an egg to form a single cell called a zygote. Occasionally two sperm meet two eggs forming fraternal twins (also called dyzygotic twins).The zygote soon develops into a small collection of cells called a blastocyst.A single egg is surrounded by a number of protective follicle cells that the sperm must penetrate to enter and fertilize the egg.
Week by week baby ultrasound image. Week by week baby ultrasound image.

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.

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Watch Baby's Growth

Sperm Meets Egg: Weeks 1 to 3 of Pregnancy

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.

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.

This Week's To-Do List