## Core Concept
Frequency in pregnancy refers to the increased urge to urinate. This symptom is commonly experienced due to the physiological changes that occur during pregnancy, particularly the mechanical pressure exerted by the expanding uterus on the bladder.

## Why the Correct Answer is Right
The correct answer, **second trimester**, is a period when the uterus has expanded enough to start exerting significant pressure on the bladder, but not so much that it elevates the bladder and urethra to a position where urinary frequency decreases, as can happen in the later stages of pregnancy. During the second trimester, the uterus rises out of the pelvis and can press on the bladder, causing an increase in urinary frequency.

## Why Each Wrong Option is Incorrect
- **Option A:** The first trimester does see some changes in urinary habits, but the frequency is more notably increased once the uterus expands significantly, which usually becomes more pronounced in the second trimester.
- **Option B:** This option is actually the correct answer but let's evaluate the rest.
- **Option C:** In the third trimester, while there can still be urinary frequency, the presenting part of the fetus (often the head) can move down into the pelvis and elevate the bladder and urethra, sometimes reducing frequency.
- **Option D:** Postpartum, the uterus returns to its pre-pregnancy size, reducing pressure on the bladder and thus urinary frequency related to pregnancy typically decreases.

## Clinical Pearl / High-Yield Fact
A key point to remember is that urinary frequency is one of the earliest symptoms of pregnancy for many women, often noticed as early as 4-6 weeks into pregnancy. However, the most significant impact typically occurs in the **second trimester** due to the reasons mentioned.

## Correct Answer: B. second trimester.