The most common cause of decreased variability in the fetal heart rate (FHR) that lasts 30 minutes or less is:

Choice A reason: Quickening is the first perception of fetal movement by the mother, usually occurring between 16 and 20 weeks of gestation. It is a presumptive sign of pregnancy, as it can be subjective and influenced by other factors such as gas or peristalsis.

Choice B reason: A positive pregnancy test is a probable sign of pregnancy, as it indicates the presence of human chorionic gonadotropin (hCG) in the urine or blood. However, it is not conclusive, as it can be affected by false positives or false negatives, or by other conditions that produce hCG.

Choice C reason: Braxton Hicks contractions are irregular, painless uterine contractions that occur throughout pregnancy. They are a probable sign of pregnancy, as they indicate uterine activity and preparation for labor. However, they are not diagnostic, as they can also occur in non-pregnant women or in response to other stimuli.

Choice D reason: Fetal movement palpated by the nurse-midwife is a positive sign of pregnancy, as it confirms the presence of a living fetus in the uterus. It is an objective and reliable sign that can be detected by a skilled examiner after 20 weeks of gestation.

Choice A reason: This is not the main reason, as the use of contraceptives for an extended time does not necessarily affect the fertility of a woman. Most contraceptives are reversible and do not cause permanent damage to the reproductive system. However, some contraceptives may take longer to wear off than others, and some may have side effects that can interfere with ovulation or implantation.

Choice B reason: This is the main reason, as the aging process can affect the ovaries and the quality and quantity of the eggs. As a woman ages, her ovarian reserve (the number of eggs in the ovaries) declines, and the eggs become more prone to chromosomal abnormalities. This can reduce the chances of conception and increase the risk of miscarriage or birth defects.

Choice C reason: This is not the main reason, as prepregnancy medical attention is not a prerequisite for achieving pregnancy. However, prepregnancy medical attention can be beneficial for a woman who is older than 35 years, as it can help identify and manage any existing or potential health problems that may affect the pregnancy, such as diabetes, hypertension, or thyroid disorders.

Choice D reason: This is not the main reason, as personal risk behaviors are not specific to a woman who is older than 35 years. Personal risk behaviors are factors that can negatively affect the fertility of any woman, regardless of age. Some examples of personal risk behaviors are smoking, drinking, using drugs, having multiple sexual partners, or having sexually transmitted infections.