During normal pregnancy, the primary metabolic derangement observed is a mild respiratory alkalosis. This is due to the stimulatory effect of progesterone on the maternal respiratory center, resulting in increased minute ventilation and a subsequent decrease in arterial carbon dioxide tension (PaCO2)(Ahmed, 2021). As a compensatory response, the kidneys increase excretion of bicarbonate, which helps maintain the pH at a slightly elevated level (typically within the normal range or mildly increased at 7.40–7.45)(Ahmed, 2022). Thus, the correct answer is:

d. Respiratory alkalosis

This physiological change is well established in human studies. In a study examining arterial and cerebrospinal fluid parameters in third-trimester pregnant women, all pregnant participants displayed hypocapnic alkalosis, further confirming that respiratory alkalosis is a predominant acid-base adaptation in late pregnancy(Zadek, 2022). Other studies reviewing the physiologic changes in pregnancy, including organ system adaptations, consistently report partially compensated respiratory alkalosis as a normal maternal adaptation(Costantine, 2014).

Severe metabolic alkalosis is uncommon in pregnancy and more typically occurs as a result of conditions like prolonged vomiting (e.g., hyperemesis gravidarum), but it is not the physiologic norm(Frise, 2013). Similarly, metabolic acidosis can be precipitated in pregnancy under pathologic circumstances such as starvation ketoacidosis, but again, this is not the expected acid-base state during an uncomplicated pregnancy(Sinha, 2014), (Abraham, 2024), (Chausse, 2018).

Specifically, metabolic acidosis and alkalosis, as well as respiratory acidosis, are occasional complications in certain pathological pregnancy states or secondary to comorbid conditions, not typical of healthy pregnancies. The physiological respiratory alkalosis may also be subject to compensation by the renal system, preventing pH from rising excessively(Ahmed, 2021), (Zadek, 2022).

Based on a broad review across studies and clinical reviews(Ahmed, 2022), (Ahmed, 2021), (Zadek, 2022), (Costantine, 2014), (Frise, 2013), (Sinha, 2014), (Abraham, 2024), (Chausse, 2018), (Albright, 2016), (Cao, 2018), (Netto, 2020):

• The predominant acid-base derangement in healthy pregnancy is mild, compensated respiratory alkalosis.

Would you like a deeper dive into the renal compensatory mechanisms or the implications of this respiratory alkalosis for fetal physiology? Or are you interested in pathological states that can change the acid-base balance in pregnancy, such as hyperemesis gravidarum or gestational diabetes? Let me know how you'd like to proceed!