Introduction to Human Development: Videos & Practice Problems

Human development begins with the process of conception, which occurs when an oocyte, or immature egg, is fertilized by a sperm cell. This moment marks the start of the development of the conceptus, a term that encompasses both embryos and fetuses without specifying their developmental stage. Understanding these terms is crucial as they lay the foundation for discussing prenatal development.

The gestational period, also known as the prenatal period, is the timeframe during which the conceptus develops. This period is traditionally measured from the first day of the mother's last menstrual period (LMP) until birth, typically spanning approximately 40 weeks. It is important to note that conception usually occurs about two weeks after the LMP, meaning that at the moment of conception, a woman is considered to be two weeks pregnant. Consequently, while the entire gestational period is 40 weeks, the actual development of the conceptus occurs over a period of about 38 weeks, starting from conception to birth.

This method of measuring pregnancy is utilized because the first day of the last menstrual period is a more easily identifiable date for women, making it a reliable reference point. However, this approach requires an understanding of the discrepancy between the gestational age and the actual developmental age of the conceptus. As you study human development, keep in mind that this chapter will focus on the 38 weeks of prenatal development, emphasizing the significant changes that occur from conception until birth.

When a woman takes a pregnancy test and receives a positive result, she may be surprised to learn that her doctor refers to her as being several weeks pregnant, even if conception seems recent. This confusion often arises from the way pregnancy is calculated. Typically, pregnancy is measured from the first day of the last menstrual period (LMP) rather than the date of conception. This method is used because it provides a more consistent and reliable reference point for most women.

To clarify, ovulation, which is when conception occurs, usually happens about two weeks after the start of the last menstrual period. Therefore, when a doctor states that a woman is four weeks pregnant, it includes the two weeks prior to ovulation when the body is preparing for a potential pregnancy. This means that for the first two weeks of the pregnancy calculation, the woman is not actually pregnant yet, but the timeline still counts from the LMP for accuracy in reporting and tracking the pregnancy's progress.

This approach is beneficial because women are generally more aware of the start date of their menstrual cycle than the exact date of ovulation, making the LMP a practical marker for estimating gestational age. Thus, understanding this method helps clarify why a woman may be considered four weeks pregnant even if she conceived only recently.

The prenatal period, also known as the gestational period, is a crucial phase in human development that is divided into three main stages: the preembryonic stage, the embryonic period, and the fetal stage. Each of these stages marks significant developmental milestones and transformations in the conceptus.

The preembryonic stage lasts from conception to approximately week 2. During this time, the conceptus is referred to as a blastocyst, which is essentially a cluster of cells that will eventually implant into the uterine wall. Key processes during this stage include fertilization, cleavage, and blastocyst formation, all occurring within the first week. Implantation, where the blastocyst embeds itself into the uterine lining, also takes place during this period.

Following the preembryonic stage is the embryonic period, spanning from week 3 to week 8. During this time, the conceptus is called an embryo. This stage is characterized by rapid development, including gastrulation, which is the formation of the three germ layers that will give rise to various tissues and organs. Organogenesis, the process of organ formation, occurs predominantly during this period, alongside the development of extraembryonic membranes and the placenta. The placenta begins to form around week 2 and becomes functional by week 12, continuing to grow throughout the fetal stage.

The final stage is the fetal stage, which extends from week 9 until birth, around week 38. During this time, the conceptus is referred to as a fetus. This stage is marked by continued growth and maturation of the organs and systems established during the embryonic period. Most significant skeletal growth and organ development occur during this time, preparing the fetus for life outside the womb.

Throughout the prenatal period, various developmental events occur in a specific timeline, with major milestones such as fertilization, implantation, and organ development occurring at distinct weeks. Understanding this timeline is essential for grasping the complexities of human development and the critical processes that take place before birth.

At 10 weeks of gestation, a developing conceptus is referred to as a fetus. Understanding the stages of prenatal development is crucial in the study of human biology. The early stages include the blastocyst phase, which occurs from conception to about 2 weeks, followed by the embryonic stage, which lasts from 3 to 8 weeks. During this embryonic period, the developing organism is called an embryo. After 8 weeks, the term shifts to fetus, which continues until birth, typically around 40 weeks. This classification is essential for recognizing the developmental milestones and the physiological changes that occur during pregnancy.