Many women who are trying to conceive, or who've missed a period, examine each symptom they experience as a potential sign of pregnancy. While some of the early pregnancy symptoms are fairly well known---morning sickness, for instance, is nearly ubiquitously recognized as a sign of pregnancy---others are a bit less widely recognized. Many women find that early pregnancy changes the smell of their urine. They say that it's possible---particularly for second-time mothers---to use urine odor as an early pregnancy sign.
Human Chorionic Gonadotropin
Shortly after a fertilized egg implants into the lining of the uterus, it begins to secrete the hormone human chorionic gonadotropin, or hCG. Dr. Gary Thibodeau, in his book "Anatomy and Physiology," explains that hCG is often called the "pregnancy hormone," since it is only found in the blood and urine of pregnant women, and its presence may therefore be taken as a positive sign of pregnancy. In fact, home pregnancy tests detect hCG. Some women are able to smell hCG in the urine, and find that the urine of early pregnancy has a strong, pungent odor as compared to their normal liquid waste. While first time mothers may not recognize this odd odor as that of hCG, during later pregnancies, women can often recognize the smell.
Pregnancy hormones such as progesterone cause a woman's nose to become much more sensitive than usual. This is helpful in many regards---it allows her to detect spoiled food, and may help guide her toward healthy food choices. Further, it helps her to avoid environmental toxins, such as cigarette smoke, since their odors become so bothersome. In their book, "You: Having A Baby," Drs. Michael Roizen and Mehmet Oz note that this increased nasal sensitivity allows women to smell things that they normally wouldn't, and allows them to detect odors in concentrations far less than those normally required to activate the nose. Though some women find that their urine smells much stronger early in pregnancy, Drs. Roizen and Oz suggest that this may be due at least in part not to a change in the urine, but a change in the nose. While the urine might smell identical to a woman's pre-pregnancy urine to the casual observer, pregnant women smell everything, even urine, to a greater extent than they did prior to pregnancy. Even though the strong smell of urine due to nasal sensitivity doesn't necessarily represent a change in the urine, note the doctors, it does represent a change in the mother---and as such, may be a sign of early pregnancy.
One of the first changes a woman experiences during pregnancy is an increase in kidney function. This is due to increased progesterone causing the heart to work harder, which in turn increases blood filtration. The more blood the kidneys filter, explains Dr. Lauralee Sherwood in her book, "Human Physiology," the more urine they will produce. Increased kidney function can change the smell of urine slightly, and since metabolism also increases during pregnancy, it's not uncommon for the urine of pregnant women to smell a bit stronger than usual---particularly if they haven't yet begun drinking more water in response to increased urination and increased thirst. A typical "urine" smell that is nevertheless somewhat stronger than normal, therefore, can be a sign of early pregnancy.