During your pregnancy, you will see many changes in your body. However, the most incredible changes are occurring in your developing baby. In 9 short months, your baby grows from a few small cells into a fully grown, fully developed little being that can live on its own outside of your body. For 9 months, your body has protected, nourished and nurtured the baby as it has grown to maturity, ready to begin its life. And although you won't be able to see much more than an enlarging abdomen, incredible things are happening inside your body.
This chapter attempts to give you an idea of some of the fascinating things that occur as your baby grows and develops. After reading it, you may agree with me that the growth of your baby is truly a miracle.
Your Baby's Due Date
Two Ways to Figure the Due Date
Most women don't know the exact date their baby was conceived, but they usually know the day their last menstrual period began. The doctor subtracts 2 weeks from the date of the last period as an estimate of when conception occurred. Your estimated due date is 38 weeks after the date of conception (40 weeks after the first day of your last period).
There is a second way to determine your due date. Add 7 days to the date of the beginning of your last menstrual period, then subtract 3 months. This gives you the approximate date of delivery. For example, if your last period began on January 20, your estimated due date is October 27.
Gestational Age and Fertilization Age
Gestational age, also called menstrual age, dates a pregnancy from the first day of the last menstrual period. It is 2 weeks longer than the fertilization age.
Fertilization age, also called ovulatory age, is 2 weeks shorter than gestational age and dates from the actual date of conception. This is the actual age of the fetus.
Baby Development During Pregnancy
Your baby grows and changes from a small group of cells to a fully developed baby ready to begin life. The designation between "embryo" and "fetus" is somewhat arbitrary. During the first 8 weeks of development (10 weeks of gestation), the developing baby is called an embryo. From 8 weeks of development until delivery, it is called a fetus. The great changes your baby goes through to become a fully developed baby are easier to follow if we look at them in each trimester.
Trimesters
The length of your pregnancy is divided into three trimesters, each about 13 weeks long.
First trimester development. The first trimester represents the greatest change for any developing fetus. In the first 13 weeks of development, your baby grows from a collection of cells the size of the head of a pin to a fetus the size of a softball. Organs begin developing, and your baby begins to look more like a baby.
Very few, if any, structures in the fetus are formed after the 12th week of pregnancy. This means your baby forms all of its major organ systems by the end of the first trimester. These structures continue to grow and to develop until your baby is born.
Second trimester development. At the beginning of the second trimester (14th week), your baby weighs less than 1 ounce (28g) and is only about 4 inches (10cm) long.
Third trimester development. Your baby weighs about 1-1/2 pounds (0.7kg) at the beginning of the third trimester (27th week), and its crown-to-rump length is under 9 inches (22cm). {Crown-to-rump length is the measurement from the top of the baby's head [crown] to the buttocks of the baby [rump].) When it is delivered, your baby will weigh close to 7-1/2 pounds (3.4kg) and be about 21 inches (53cm) long.
Baby's Size and Weight
Birthweight varies greatly from baby to baby. However, the average weight of a baby at term is 7 to 7-1/2 pounds (3.3 to 3.4kg).
Ultrasound is the test of choice to estimate fetal weight. A formula has been established to help estimate fetal weight using this technology. Several measurements are taken, including the diameter of the baby's head, circumference of the baby's abdomen and length of the femur (thighbone) of the baby's leg. Occasionally other fetal measurements are taken. A drawback of using ultrasound for estimating fetal weight is that estimates may vary as much as half a pound (225g) in either direction. However, the accuracy of predicting fetal weight with ultrasound continues to improve.
The size of the fetus's head surprises many of my patients. When you are 13 weeks pregnant, your baby's head takes up about half the crown-to-rump length (measurement from top of the head to the baby's buttocks). In 2 months, when you are 21 weeks pregnant, the head will be about one-third of the fetal body. At birth, your baby's head will be one-fourth the size of its body.
Your Baby's Heart
Your baby's heart starts beating very early. By the 6th week of pregnancy (age of fetus is 4 weeks), the heart tubes fuse and contractions of the heart begin. This can be seen on ultrasound.
Occasionally you may listen to your baby's heartbeat at the doctor's office and hear it skip a beat. An irregular heartbeat is called an arrhythmia. Arrhythmias in a fetus are not unusual, so don't be overly concerned. The equipment could be faulty or there may be some other problem transmitting the sound.
Arrhythmias are not usually serious in a baby before birth; many disappear after the baby is born. If an arrhythmia is discovered before labor and delivery, you may require fetal heart-rate monitoring during pregnancy. When an arrhythmia is discovered during labor, it may be decided to have a pediatrician present when the baby is born.
Your Baby in the Womb
Some patients are startled when they see an ultrasound of their baby and it appears that the baby has its mouth open. In addition to turning over and moving their hands and feet, babies do open and close their mouths in the womb. The fetus may also suck its thumb or finger.
Digestive System Develops
By 21 weeks, the fetal digestive system has developed enough to allow the fetus to swallow amniotic fluid. The fetus absorbs much of the water in the swallowed fluid. Hydrochloric acid and adult digestive enzymes are present in small amounts in the fetal digestive system at 21 weeks.
Researchers believe swallowing amniotic fluid may help growth and development of the fetal digestive system. It may also condition the digestive system to function after birth. By the time a baby is born, he or she may swallow large amounts of amniotic fluid, as much as 17 ounces (500ml) of amniotic fluid in a 24-hour period.
Eyes Open
Your baby can open its eyes inside the uterus. Eyelids cover the eyes and are fused or connected around 11 to 12 weeks. They remain fused until about 27 to 28 weeks, when they open.
Hearing Begins
A baby can hear inside the womb, before it is born. Life inside the womb may be like living near a busy freeway. The developing baby hears a constant background of digestive noises and the maternal heartbeat. The mother-to-be's voice is also heard, although the fetus may not hear higher-pitched tones. There is evidence that by the third trimester the fetus responds to sounds it hears. Researchers have noted fetal heart-rate increases in response to tones it hears through the mother's abdomen. Newborns have been found to prefer their mother's voice to a stranger's, which suggests they recognize the mother's voice. They have also been found to prefer their mother's native language, and they respond strongly to a recording of an intrauterine heartbeat.