During Irina's first week of pregnancy, __________.

Correct Answer: B

Rationale: During the first week of pregnancy, the zygote (fertilized egg) undergoes rapid cell divisions and forms a blastocyst. This is a crucial stage where the cells differentiate and specialize, setting the foundation for the development of the embryo. It is during this time that the zygote implants itself into the uterine wall, initiating the process of pregnancy. Choices A, C, and D are incorrect because during the first week of pregnancy, the focus is on the initial development and multiplication of the zygote, not on the development of heart muscles, a primitive brain and spinal cord, or the embryo's ability to move.

Correct Answer: C

Rationale: During the last half of the first month of pregnancy, the nervous system develops the fastest among the options provided. This rapid development of the nervous system is crucial for the overall growth and functioning of the fetus. The nervous system includes the brain and spinal cord, which are essential for various bodily functions and will continue to develop throughout the pregnancy. The circulatory system, although important, does not develop at the fastest rate during this stage. The endocrine system and skin also play significant roles in the development of the fetus but are not the systems that develop the fastest in the last half of the first month of pregnancy.

Correct Answer: B

Rationale: The correct answer is the third month of pregnancy (choice B). During the third month of pregnancy, which is around 9-12 weeks, a fetal heartbeat can typically be heard using a stethoscope. This is an exciting milestone for expectant parents as it provides reassurance about the baby's well-being. Choices A, C, and D are incorrect because by the fourth week of pregnancy, the fetal heartbeat is not usually detectable with a stethoscope. The end of the second trimester and the beginning of the third trimester are too late in the pregnancy timeline for this milestone.

Correct Answer: B

Rationale: A baby born between the seventh and eighth months of pregnancy is considered premature. Premature babies are born before they are fully developed and may need assistance with breathing, including oxygen. They are at a higher risk for complications compared to full-term babies. Choice A is incorrect because premature babies are at a higher risk for health issues compared to full-term babies. Choice C is incorrect as modern medical advancements have significantly increased the survival rate of premature babies. Choice D is incorrect as being small-for-date does not solely depend on the gestational age at birth.

Correct Answer: D

Rationale: The harm done by teratogens depends on the age at the time of exposure. This means that the effects may vary depending on the developmental stage of the fetus when exposed to the teratogen. Teratogens can have different impacts at different stages of pregnancy, leading to a range of potential outcomes in terms of physical and/or cognitive development. Choices A, B, and C are incorrect because the harm caused by teratogens is not straightforward, may not be apparent at birth, and can go beyond physical damage, affecting cognitive development as well.