The fourth month of pregnancy (weeks 14 to 17)
At 14 weeks, the baby measures 7.6 cm (3 inches) in length and weighs less than 28 g (1 oz). This month, the baby's body will grow quickly. The lower limbs are a bit less developed and shorter than the upper limbs.
By the end of week 14, the external genitals are well developed and it is possible to determine if the baby is a boy or girl! Small reflex movements can occur, although they are still too light to be felt by the mother. Primary centres of bone development appear in the skeleton, especially in the skull and long bones. The circulatory system is operating and the liver starts producing bile.
In week 15, the skin is very thin and the blood vessels underneath are visible. At this point, the heart pumps about 24 litres, or 25 quarts, of blood per day!
By week 16, the unborn baby is completely formed. Limb movements become coordinated by the end of this week, but they are still too slight to be felt by the mother. The baby may start to slowly move her eyes. Fingernails start to form during this week. The bladder is formed and it empties itself every 40 minutes or so. It is now possible to use ultrasound to try to distinguish the sex of the baby. However, determining the sex of the baby this way is not always reliable.
After week 17, no new structures appear. The baby may start to show startle reflexes – with outstretched arms and legs – in response to loud noises. The baby is moving around in the uterus and touching her toes and face.
The fifth month of pregnancy (weeks 18 to 21)
This month, the baby continues to grow. By this point, the head is relatively small in proportion to the rest of the body and the lower limbs have become longer.
During this period, the baby is covered with fine downy hair called lanugo. Hair begins to grow on her head. Eyebrows start to appear.
Bone development is active during this period, and bones are clearly visible by the end of week 18. In addition, tooth buds have appeared, and the fingers and toes are well defined. By the end of week 18, the baby starts to suck and swallow, and is now nourished by the placenta. Though human-looking, the baby still cannot survive outside the uterus at this point.
At 19 weeks, the skin is thin because there is very little underlying fat. By 20 weeks, the skin is covered with a greasy coating known as vernix. Vernix protects the skin from exposure to the amniotic fluid, which is fluid that surrounds and protects the baby. The lanugo helps to hold the vernix on the skin. Brown fat starts to form, mostly around the neck, the breastbone, and the area surrounding the kidneys.
By week 20, ovaries in the female baby contain egg cells, the uterus is formed, and the vagina has begun to develop.
The sixth month of pregnancy (weeks 22 to 26)
During this period, your baby will gain a substantial amount of weight. By 26 weeks, she measures about 30 cm (12 inches) in length and weighs about 820 g (1 lb 13 oz). Although still somewhat lean, your baby has been gaining brown fat and is now better proportioned. The brown fat also helps the baby retain body heat.
The skin is usually reddish, wrinkled, and translucent at this point. Blood is visible in the tiny blood vessels known as capillaries. Around this time, the baby’s heartbeat can usually be heard with an ordinary stethoscope. At 25 weeks, rapid eye movements begin. The eyelids start to part, and the eyes open.
By week 26, the lungs have begun to secrete a chemical called surfactant, which is essential to the development of the tiny air sacs within the lungs. Although several organ systems are functioning at this time, the respiratory system is not well developed yet. A baby born during the sixth month of pregnancy has a chance of surviving with intensive care, but it may die due to its immature respiratory system. If a baby is expected to be born prematurely at this time, it might be possible to give the mother a steroid shot to enhance lung development.
The baby is now aware of light and dark. She looks more like a "real" baby, albeit a bit wrinkly from the amniotic fluid.
You will be very aware of your baby’s movements and may be able to appreciate her wake and sleep patterns. These are patterns of vigorous activity followed quiet times. You may want to keep track of these patterns, as they may continue after your baby is born. The end of this month will be an exciting time, because your partner and other people will be able to see the baby moving!
It is recommended that you keep track of your unborn baby’s kick counts from about six months of pregnancy, as a way of making sure that she is OK. Each day, time how long it takes for your baby to make 10 kicks, flutters, swishes, or rolls. You should feel at least 10 movements within two hours, but you will probably feel that many movements in a much shorter amount of time.
Use a notebook to record the time that you feel each kick, flutter, swish, or roll, until your baby has made 10 movements. You may start to notice patterns and a general length of time that baby usually takes to make 10 movements. If you notice major deviations from the pattern, check with your health care provider.
For more information, see "Kick Counts."