ch. 3

Prenatal Development

Introduction

Prenatal development refers to the process by which a human embryo and later fetus develops during pregnancy, from conception to birth. This period is critical for understanding how genetic and environmental factors influence later psychological and physical outcomes. The following notes summarize key stages and concepts in prenatal development, including conception, the germinal period, and the embryonic period.

Conception and Fertilization

Conception

• Conception occurs when a sperm cell from a male fertilizes an egg cell (ovum) from a female, typically in the fallopian tube.

• Females typically release one egg per menstrual cycle (about every 28 days), triggered by hormonal changes involving follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

• Infertility can result from various factors, including ovulation problems, blocked fallopian tubes, or sperm issues.

• Assisted Reproductive Technologies (ART) such as in vitro fertilization (IVF) and artificial insemination are used when natural conception is difficult.

Example: IVF involves fertilizing an egg outside the body and implanting the embryo into the uterus.

Hormonal Regulation and Ovulation

• Ovulation is triggered by a spike in LH and FSH, leading to the release of an egg.

• Body temperature rises slightly during ovulation, which can be used to track fertility.

Success Rates of ART by Age

Success rates for ART procedures such as IVF decrease with maternal age. The following table summarizes the chances of giving birth to a single baby at term and at any time, based on maternal age and embryo source.

Age of Woman	< 35 Years	35-37 Years	38-40 Years	> 40 Years
Woman's Own Embryos, Fresh (% Born at Term)	21%	17%	11%	4%
Woman's Own Embryos, Fresh (% Any Live Birth)	33%	29%	17%	9%
Woman's Own Embryos, Frozen (% Born at Term)	31%	30%	27%	21%
Donor Embryos, Fresh or Frozen (% Born at Term)	25%	25%	28%	28%
Donor Embryos, Fresh or Frozen (% Any Live Birth)	46%	41%	40%	39%

Source: Centers for Disease Control and Prevention (2018)

Stages of Prenatal Development

Prenatal development is divided into three main periods:

1. Germinal Period (first 2 weeks after conception)

2. Embryonic Period (weeks 3-8)

3. Fetal Period (week 9 to birth)

The Germinal Period

• Lasts from conception to about 2 weeks.

• Begins with fertilization and ends with implantation of the blastocyst in the uterine wall.

• Rapid cell division occurs, forming a blastocyst (about 150 cells).

• Implantation is a critical step; only about half of fertilized eggs are successfully implanted.

• Miscarriage is common during this period, often before a woman knows she is pregnant.

Day	Event
1	Fertilization usually occurs within 24 hours of ovulation.
2-4	The zygote divides and begins a 3-4 day journey down the fallopian tube to the uterus.
5	Blastocyst forms; cells begin to specialize.
6-7	Blastocyst attaches to the uterine wall (implantation).
8-14	Blastocyst embeds in the wall of the uterus; placenta begins to form.

The Embryonic Period

• Extends from the third to the eighth week after conception.

• Major organs and body systems begin to develop (organogenesis).

• Three layers of cells form: ectoderm (nervous system, skin), mesoderm (muscles, bones, circulatory system), and endoderm (digestive system, lungs).

• The placenta and umbilical cord develop, providing nutrients and removing waste.

• The embryo is especially vulnerable to teratogens (harmful substances) during this period.

Week	Event
3	The embryo is about 1.5 mm long; the neural tube (future brain and spinal cord) begins to form.
4	Heart starts beating; arm and leg buds appear; eyes, ears, and mouth begin to form.
5	Major structures and organs are present; embryo straightens and assumes a more human appearance.

Neural Tube Development

• The neural tube forms from the ectoderm and will become the brain and spinal cord.

• Failure of the neural tube to close properly can result in defects such as spina bifida or anencephaly.

• Folic acid supplementation before and during early pregnancy reduces the risk of neural tube defects.

Example: Spina bifida is a condition where the spinal cord does not close completely, leading to physical and neurological problems.

Key Terms and Definitions

• Zygote: The single cell formed by the union of sperm and egg.

• Blastocyst: A hollow ball of cells formed early in development, which implants in the uterine wall.

• Embryo: The developing organism from implantation to the end of the eighth week.

• Placenta: The organ that connects the developing fetus to the uterine wall, allowing nutrient uptake and waste elimination.

• Teratogen: Any agent that can cause harm to the developing embryo or fetus (e.g., drugs, infections).

Summary Table: Stages of Prenatal Development

Stage	Time Frame	Main Events
Germinal	Weeks 1-2	Fertilization, cell division, implantation
Embryonic	Weeks 3-8	Organogenesis, neural tube formation, vulnerability to teratogens
Fetal	Week 9 to birth	Growth and maturation of organs and systems

Additional info:

• Psychological research on prenatal development often focuses on how early biological and environmental factors influence later cognitive, emotional, and behavioral outcomes.

• Miscarriage is most common in the first trimester, often due to chromosomal abnormalities.

• Folic acid is recommended for women of childbearing age to prevent neural tube defects.