When the blastocyst burrows deep into the uterine lining, __________.

Correct Answer: A

Rationale: When the blastocyst burrows deep into the uterine lining, this process is known as implantation. Implantation is a crucial step in the early stages of pregnancy where the blastocyst attaches to the uterine wall in order to receive nourishment and continue developing. Choice B is incorrect because implantation is a necessary step for pregnancy to occur. Choice C is incorrect as implantation marks the beginning of further growth and development. Choice D is incorrect as the duplication of cells occurs during earlier stages of development, not during implantation.

Correct Answer: B

Rationale: By the end of Greta's first month of pregnancy, her embryo is about ¼ inch long. During the first month of pregnancy, the embryo is in the very early stages of development, and its size is typically around ¼ inch as it continues to grow and develop in the uterus. Choices A, C, and D are incorrect because sensory abilities like sensing the world, sucking thumb, and curling toes develop later in the pregnancy as the embryo progresses through different stages of development.

Correct Answer: D

Rationale: Lanugo helps the vernix stick to the skin. Vernix is a waxy substance that covers the fetus's skin during pregnancy. Lanugo is a fine, downy hair that helps to anchor the vernix to the skin, providing protection and nourishment to the developing fetus. Choices A, B, and C are incorrect as they do not accurately describe the role of lanugo in prenatal development.

Correct Answer: D

Rationale: In the eighth month of pregnancy, lanugo typically is shed. Lanugo is the fine, downy hair that covers a fetus's body to help regulate body temperature while in the womb. By the eighth month, many fetuses shed this hair as they prepare for birth. Choices A, B, and C are incorrect because fetal growth doubling, neuron production increasing, and the formation of eyes, ears, nose, jaw, and neck occur in earlier stages of pregnancy, not specifically in the eighth month.

Correct Answer: A

Rationale: In the germinal period of prenatal development, which lasts for about the first two weeks after fertilization, the developing organism is a zygote that has not yet implanted in the uterus. During this period, teratogens rarely have any impact because essential implantation and early cellular division processes are occurring. Teratogens are more likely to have significant effects during the later embryonic and fetal periods of development. Therefore, the correct answer is A. Choice B, embryonic period, is incorrect because teratogens have a higher likelihood of impacting development during this period. Choice C, fetal period, is also incorrect as teratogens can impact development during this stage as well. Choice D, prenatal, is a broad term encompassing the entire period of development from conception to birth and is not a specific developmental stage where teratogens have a minimal impact.