The gestation of human pregnancy is approximately 38 to 42 weeks from the first day of the mother's last menstrual period, or roughly 266 days from conception. The fetus grows in a double-layer membrane sac filled with amniotic fluid during most of its time in the womb. A saucer-shaped placenta fused to the membranes and attached to the wall of the uterus receives maternal blood, where fetal blood can exchange wastes for nutrients and oxygen via the umbilical cord. This system sustains the growing fetus until birth, when newborn lungs and the digestive system take over.
Pre-Embryonic Period
By day three after fertilization, a ball of 12 dividing cells floats in the womb, nourished by secretions from the mother's body. The blastocyst attaches to the upper rear wall of the uterus and differentiates into inner and outer layers. Finger-like projections begin the process of anchoring, and dividing cells create different types of tissues.
In the second week, cells organize to create a yolk sac, which will sustain the developing embryo until day 12 when the placenta and umbilical cord begin taking shape. A second yoke sac will become the gastrointestinal tract. By 14 days, the embryo looks like two bubbles dangling from a cord. It weighs less than an ounce.
Embryonic Period
All major internal and external structures form between three and eight weeks after conception. On day 15, the mother misses her period. Blood circulation and heartbeat begin on day 20.
By the 24th day there is a clear head and tail. The neural tube closes around day 28. During week five rapid growth of the brain enlarges the head. In the sixth week upper limbs and eyes appear. The embryo has a reflex response to touch. In week seven, webbed fingers form. By the end of week eight heart development is complete.
First Trimester
Although the first trimester begins with the last menses, week nine is the beginning of the fetal period. The fetus will startle at loud noise and can hiccup. Heartbeat is detectable at 10 weeks. In an article published in "Psychology Today," Janet L. Hopson writes of the developing fetus, "at week ten, it moves its arms, 'breathes' amniotic fluid in and out, opens its jaw, and stretches. Before the first trimester is over, it yawns, sucks, and swallows, as well as feels and smells."
Second Trimester
Between weeks 13 and 15, eyelashes, eyebrows and fingerprints form. Between 16 and 18 weeks, most mothers feel kicking. The fetus is 6 1/2 inches long and weighs 12 oz. At 20 weeks, the gender can be determined. Hearing is well-developed. At 21 weeks, the fetus has rapid-eye movement sleep, associated with dreaming. By 24 weeks the baby sucks its thumb and responds to touch through the mother's abdomen.
Third Trimester
In the last 12 weeks, fat forms under the baby's skin. At 30 weeks the pupils respond to light. Lungs are more mature at 31 weeks and fetuses born then usually survive. The majority of babies turn head down by 34 weeks. By 36 weeks, breathing and sucking reflexes are ready for extra-uterine life. Growth slows and the baby prepares for birth.
References
- "Holistic Midwifery," Anne Frye, 1998
- "Psychology Today," Fetal Psychology, Janet L. Hopson, Sept/Oct 1998
