Final answer:
Expectant fathers typically think of the baby as being a fetus starting around nine weeks into the pregnancy. The fetal stage is marked by significant growth and development, with the organs maturing by the 38th week. By the end of the pregnancy, the fetus is ready to be born, with labor usually beginning around 40 weeks.
Step-by-step explanation:
Expectant fathers often begin thinking of the developing baby as a fetus starting at around nine weeks of the pregnancy. At this stage, the embryo transitions into what is known as the fetal stage. The fetus, which is about the size of a kidney bean, begins to show the recognizable form of a human being as it loses its tail-like structure. Around developmental weeks 21-30, the fetus undergoes significant growth and development, including rapid weight gain essential for maintaining body temperature after birth and the development of body features such as eyelashes. The fetus's organs, including the lungs, which produce surfactant, and the brain, which undergoes myelination, continue to mature throughout the pregnancy.
By the end of the 38th week, most of the organs are mature, and the fetus is ready for birth, although birth usually occurs around 40 weeks of pregnancy. The final weeks see substantial weight gain as the fetus becomes ready to survive outside the womb. Expectant fathers, and mothers alike, eagerly anticipate the arrival of the newborn during the last trimester, as the baby at this stage is fully developed and the birth process, or labor, is imminent.