Since most women don't know the exact date of conception, the standard method for charting a pregnancy is by counting forward from the first day of your last menstrual period (LMP). This method assumes that you ovulate 14 days after your period begins (and that you'll remember the day your period started!). These results give health care providers—and you—a ballpark estimate for your baby's due date.
For more accurate pregnancy dating, health care providers can request an ultrasound. Careful measurements taken in the first trimester are even more accurate at predicting an embryo's age than ultrasounds performed later in pregnancy. Using these measurements, a sonographer can pinpoint a baby's age to within a few days. As the baby-to-be grows larger, these measurements are less reliable at predicting a baby's age because of varying growth rates. Just like kids come in different shapes and sizes, in utero babies can grow at different rates, too.
You might already be familiar with ultrasounds from popular TV shows or from attending exams with expectant friends. These transabdominal ultrasounds are performed by placing cool gel across the belly and using a transducer, which emits sound waves, to generate a picture of your developing baby. Later in pregnancy, this method works well. But early in pregnancy, when the embryo is still tiny, the sonographer will most likely need to do a transvaginal exam.
With a transvaginal ultrasound, the sonographer places the transducer into the vagina. The transducer will not only be able to grab pictures of your growing baby, but it will also produce images with greater detail than with a transabdominal exam.
Early, accurate dating of a baby's age and expected delivery date might help your health care provider make decisions about how to manage your pregnancy. For example, for women who go into labor early -- or late -- your provider will have an accurate picture of your baby's age. And this early peek might reveal possible genetic abnormalities. Accurate dating is also important for the timing and interpretation of some specialized prenatal tests, such as the quadruple screen, nuchal translucency measurements, and others used to assess the likelihood of genetic abnormalities.
By now, your baby's heart rate is even faster than it was last week, and his heart tube has formed small bulges. His cells are frantically churning out the building blocks for what will become 40 pairs of muscles and 23 pairs of vertebrae radiating from his spinal column. Your baby's limb buds are starting to grow; they look like tiny flippers at this point. His outer ears are beginning to take shape, and he now has joints in his elbows, wrists, and knees. His eyelids are nearly complete too.
Although your baby is a mere 1/2 inch long and still curved as tightly as a comma, he's growing quickly. Some of the most important growth is happening in his brain, where new nerve cells are forming at the astounding rate of 100,000 per minute. As your baby's brain cells multiply, they will branch out and connect, forming the first primitive pathways for his central nervous system. By the time your baby is born, his brain will have more than 10 million intertwined nerve cells!
His brain is forming into three distinct parts. The first, the forebrain, is the largest section, which will later become the cerebrum, thalamus, and hypothalamus (the brain's nerve center). The midbrain, as the name implies, sits between the fore- and hindbrain, connects to the brain stem and the spinal cord. Finally, the hindbrain at the back of the head contains the cerebellum and other structures that guide unconscious bodily functions, such as breathing. Each of these delicate and interconnected brain sections is just starting to form.
As her brain develops inside her head, her facial features are taking shape on the outside. You wouldn't be able to tell if she's got grandpa's ears just yet, but her ear structures are beginning to form. In fact, all of her body parts are starting to grow: Where there were only buds, now your baby has paddles for arms and places where her fingers are forming.
Transabdominal ultrasound: An external examination to create a sonogram image of the uterus's interior by placing the imaging device, or transducer, on the lower abdomen.
Transvaginal ultrasound: An internal examination to create a sonogram image of the uterus's interior by placing the imaging device, or transducer, into the vagina.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org)