First Trimester: Images of Your Developing Baby

There's no ultrasound image of your baby-to-be for weeks 1 and 2. While your health care provider counts these two weeks toward your due date, you aren't really pregnant. Confused? Your pregnancy due date is calculated using the first day of your last menstrual period (LMP). Obviously you weren't pregnant at that time, but it's the best reference your health care provider has for estimating baby's arrival day (until you get an ultrasound, which may provide a more accurate due date).

Fetal Size: Not measurable.

Fetal Development Milestones: Fertilization!

What You're Seeing: 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.

Fetal Size: Not measureable.

Fetal Development Milestones: Positive pregnancy test!

What You're Seeing: The small circle at the center of the sonogram may not look like much, but that little sac is a kind of baby cocoon called a gestational sac. The cells that make up this sac will begin to specialize. Some cells will become part of the placenta. Some will form the amniotic sac that will fill with fluid to cushion your developing baby. Other cells are destined to form everything from delicate eyelashes to muscles and skin. But that's still a long way away.

Fetal Size: 1/18 to 1/16th of an inch (about the size of the pen dot).

Fetal Development Milestones: Cells that will form the heart and the central nervous system are developing.

What You're Seeing: The dark area is the fluid filling the gestational sac. Eventually, this fluid will be replaced by a sac containing the amniotic fluid your baby-to-be will live in for the next few months. The white circle within the fluid is called the yolk sac. Before the placenta is fully formed, the yolk sac plays a role in providing all the nutrients your baby-to-be needs to grow. Adjacent to the yolk sac, little + marks show a very early embryo. The sonographer measures the length of the embryo (the crown-rump length or CRL) to confirm or revise the due date estimated from your LMP, or to evaluate the embryo's growth.

Fetal Size: 1/6 to 1/4 of an inch.

Fetal Development Milestones: Baby-to-be takes on a tucked, C-shape. Head, legs, and umbilical cord are forming. Blood is pumping through the heart.

What You're Seeing: In this 3D image of the developing embryo, you can see a big change since previous week of the first trimester. The baby-to-be curves inward, with the umbilical cord in the middle. The head appears at the upper right side of the image. Small buds can be seen where the arms and legs will eventually develop.

Fetal Size: 1/6 to 1/4 of an inch.

Fetal Development Milestones: There's a measureable heartbeat.

What You're Seeing: Here, the sonographer demonstrates the developing baby's heartbeat. The top part of the image shows placement of a measuring tool on the ultrasound machine called an M-mode through the image of the beating heart. This tool shows movement over time, which is displayed on the bottom part of the image. The image on the bottom shows how the baby's heart rate is calculated.

Fetal Size: 1/4 to 1/3 of an inch long, or about the length of your pinkie fingernail.

Fetal Development Milestones: Head growing larger, and structures that will form the brain can be identified. Nostrils and lenses of the eyes develop.

What You're Seeing: During this week of the first trimester, you can see baby-to-be is developing in a bubble within the gestational sac. The bubble around the embryo is the amniotic cavity filled with amniotic fluid. This liquid environment gives your baby room to grow and develop and to move. The amniotic fluid also cushions your baby-to-be from any external pressure on the abdomen. The black area inside the head is part of the developing neural tube.

Fetal Size: Length, 0.6 inches; weight, 0.04 ounces.

Fetal Development Milestones: Baby's hands and feet are developing. Fingers are beginning to form, but are still fused together. Elbows and ears taking shape. Baby-to-be's body, arms and legs are getting longer. Small, jerky movements (seen on sonogram).

What You're Seeing: In this image, the embryo is lying on her back with her head to the right of the screen. In this now familiar c-shape, you can see that the baby-to-be's head is becoming larger during this part of the first trimester to accommodate her growing brain. Her brain is divided into three main parts: the forebrain, the midbrain and the hindbrain. As in the previous week, the hindbrain may be seen as a dark area in the back of the embryo's head.

Fetal Size: Length, almost 3/4 of an inch; weight, 2 grams.

Fetal Development Milestones: Facial features like eyelids and ears continue to develop.

What You're Seeing: The embryo appears at the bottom of the image with his head on the left. The arms and legs aren't seen from this angle, but the umbilical cord can be seen extending from the baby's abdomen on its way to the placenta. The sonographer has marked the embryo's crown-rump length (CRL), which will help to confirm or revise the due date estimated from the LMP. Amniotic fluid (the dark area) surrounds the developing baby.

Fetal Size: Length, about 3/4 of an one inch; weight, 2 grams.

Fetal Development Milestones: Baby's forehead is large, and the chin is underdeveloped. Baby's toes are fused together.

What You're Seeing: This image gives you a sneak peek at the interaction between the mother and baby during the first trimester. The embryo is lying on its back with his head on the right side. His heart is the blue area. The umbilical cord stretches from the developing baby's abdomen to the placenta, and the red and blue colors within the cord represent blood going to and from the placenta, where it picks up oxygen and nutrients.

Fetal Size: Length, 1.22 inches; weight, 0.14 ounces.

Fetal Development Milestones: Eyelids are developing, and Baby's ears are fully formed (but not yet in position). The neck is forming. Fingers and toes are becoming more defined.

What you're seeing: You'll notice in this image that your baby-to-be is looking more and more like a newborn. Her arms and legs are visible, and a recognizable profile can be seen. The bright white areas in the profile are facial bones.

Fetal Size: Length, 1.22 inches; weight, 0.14 ounces.

Fetal Development Milestones: Rudimentary forms of all the organs are present, and cartilage is beginning to ossify and turn into bone. At the end of this week, your embryo becomes a fetus.

What You're Seeing: This 3-D image of your developing baby shows how lifelike she appears at this early age. Notice that baby-to-be is tucked into a c-shape, with her head toward her stomach and her arms and legs jutting outward. The umbilical cord is seen going from the baby's abdomen to the placenta.

Fetal Size: Length, 1.61 inches; weight, 0.25 ounces.

Fetal Development Milestones: Chin and neck are developing. Facial features are becoming more defined. Baby's ears move higher on the head.

What You're Seeing: Baby-to-be is lying on her back with her head on the left side of the image and her legs pointing up. From this image, you can see that her neck is growing, separating her large head from the rest of her body. Her head still makes up more than 50% of her body size, which is normal. Facial bones are again seen as bright white areas in the profile.

Fetal Size: Length, 1.61 inches; weight, 0.25 ounces.

Fetal Development Milestones: Fingers and toes are now visible on an ultrasound. Baby's genitalia are forming but not visible by ultrasound.

What You're Seeing: In this 3-D picture, notice that Baby's delicate facial features are more visible. Muscles and bones are building in baby's arms and legs. The baby has slung the umbilical cord over one shoulder. A close look also reveals tiny fingers and toes. If the image were live, you would be able to see the developing baby's jerky movements.

Fetal Size: Length, 2.13 inches crown to rump; weight, 0.49 ounces.

Fetal Development Milestones: Fingernails and toenails are beginning to form. Genitalia is continuing to develop as well, although it isn't visible on ultrasound. The kidneys are beginning to function. And baby may be sucking her thumb!

What You're Seeing: With the baby in profile and the head on the right side, you can see that the facial profile is becoming more and more like what you'd expect to see in a newborn. The developing baby has one hand in front of the face as if he's shading his eyes.

Fetal Size: Length, almost 3 inches crown to rump; weight, almost 1 ounce.

Fetal Development Milestones: Kidney and urinary tract are functioning. Baby's fingerprints have formed and she continues to suck her thumb. Tooth buds are now developing.

What You're Seeing: In this profile shot, notice that the baby-to-be is lying with her bottom on the left-hand side of the image and her head to the right. (Even though the fetus is referred to as "she" here, the sonographer may or may not be able to identify the baby's gender at this point.) Her legs are clearly visible raised up, knees bent. The line across the middle of the profile is the sonographer's measurement of baby's crown-rump length (CRL). With this measurement, the sonographer is able to determine your baby's age.

Copyright &copy 2009 Meredith Corporation.

All ultrasound images for this slideshow were provided by the sonographers of the Johns Hopkins Maternal-Fetal Diagnosis and Treatment Center. We are grateful to Christine Bird, BS, RDMS, RVT, chief obstetrical sonographer, and Jude Crino, M.D., medical director, for their assistance with this project.

For examples of prenatal ultrasounds and more information on your baby's fetal development, be sure to visit www.aium.org and www.marchofdimes.org.

Finished with your first trimester? Click here for additional prenatal ultrasounds:

For more information about ultrasounds and fetal development, check out the following resources: