Best Way to Understand Prenatal Development

Life inside the womb is fas­cinating! The process of development from the moment of con­ception to the moment of delivery is divided into three phases:

• The ovum. This phase lasts about ten to fourteen days; it is the period from the moment when the nucleus of the sperm fuses with the nucleus of the ovum until the fertil­ized ovum becomes firmly implanted in the wall of the uterus. From then on, the ovum is dependent on the moth­er, and the next phase of prenatal life begins.

• The embryo. Progress is rapid in subsequent weeks. The embryo becomes surrounded by membranes, which form a sac filled with a watery liquid. The embryo floats in this liq­uid, but is attached to the sac by the umbilical cord, which itself is attached to the placenta. The placenta acts as a fil­ter for the embryo (and fetus). During this period, which lasts until about eight weeks after conception, an embryo starts to develop many major organs, including eyes, ears, the mouth (which can open and close), limbs, and the all-important spinal cord.
• The fetus. This third period of growth lasts from about two months until the birth itself, and sees further development of body systems already laid down during the embryonic period. By twelve weeks, muscles are well developed, eyelids and lips are present, toes and fingers are observable, and the sex can be identified. Around the end of the sixteenth week, a mother may report that she feels her fetus moving. The fetus is now more than four inches long, and the ears are formed. By twenty-four weeks, eyes are completely formed (but closed), and taste buds appear on the tongue. A fetus is capable of surviving if born at this point.

Knowledge of fetal capabilities from the twenty-fourth to the fortieth week has been compiled from two sources. First, studies of premature babies born from the twenty-fourth week onwards have been able to plot changes in the senses of the fetus. By week twenty-four, a baby’s nervous, respiratory, and other bodily systems have advanced to such an extent that it can survive outside the womb, with special help. As a result, we now know, for instance, that a fetus born at

• twenty-eight to thirty-three weeks will make mild attempts to avoid bright lights and loud noises, may have a weak cry, and will have some movements;
• thirty-three to thirty-six weeks has stronger movements, a stronger response to bright lights and loud noises, and will give a good cry when hungry;
• thirty-six to forty weeks will begin to track movements of objects with its eyes, will be active, and will have a very solid cry when distressed.

The sensory abilities of premature babies under forty weeks are not developed to the same extent as those of full-term babies. For instance, premature babies’ sense of touch is not so highly attuned, and they are not very sensitive to pain. Yet they do respond to other types of sensory stimulation. Research has confirmed that babies born from twenty-eight weeks onwards can differentiate basic tastes such as sweet, sour, salt, and bitter, and basic odors.

A second source of knowledge about the sensory skills of an unborn fetus comes from the widespread use of ultrasonogra­phy. Examinations carried out with this technique demonstrate that a fetus spends a great deal of time moving around the womb. And since these movements tend to be coordinated, this probably means the baby has a good sense of balance. Ultrasound techniques have shown that an unborn child will give a startle response to a loud noise. Eye movements have also been detected.

With this level of sensitivity, it is hardly surprising that a fetus can be affected by stimulation outside the womb. The fact that an unborn fetus at twenty-eight weeks responds to noise does not mean that it hears these noises in the same way as an already-born baby of similar maturity. Remember that an unborn fetus is surrounded by fluid, which is not a very good transmitter of sound. Think for a moment about swimming in your local pool. The pool area may be very noisy, with lots of people shouting to each other and splashing around. The sound echoes loudly from one wall to another. However, as soon as you put your head under water, sounds become muffled. Submerged in water, you have great difficulty picking out indi­vidual sounds and specific voices. Scientists believe that’s what it is like for a baby surrounded by liquid in the womb.

In addition, the outer ears of a fetus are covered by a creamy material known as vernix, which acts as a further barrier to clear hearing inside the womb. Studies have also shown that although an unborn baby may be startled at first by a loud noise (indicat­ed by an increase in fetal heartbeat), the effect soon fades.

There is no direct connection between the mother’s nervous system and that of her fetus. Yet, as early as the 1940s, scientists proved that a mother’s emotional condition is indirectly con­nected to her fetus’s physiology. This happens because emo­tional arousal—such as anger or fear—causes a physical reaction that releases chemicals into the bloodstream. Hormones are also secreted. These biochemical changes are transferred through the placenta, causing the fetus to become agitated. Fetal movements in the womb increase many times over when the mother experiences emotional stress. If the period of exci­tation lasts for weeks, the fetus may persist with this higher rate of movement for the remainder of the pregnancy.