How Come Baby Bumps Can Look So Different From One Woman to Another?

Women who are newly pregnant tend to spend a lot of time thinking about when they're going to "pop" (or finally start showing). But there's no one-size-fits-all pregnancy, and for some mamas-to-be, a baby bump appears pretty quickly—while others bear a flat tummy well into their second trimester.

Don't believe it? Look no further than Sarah Stage, a 33-year-old model who recently announced she's expecting baby number two: Six months into her pregnancy, her stomach still looks taut and toned.

Meanwhile fitness star Emily Skye, who's also expecting a baby, says some people have been commenting that her belly looks bigger than it should be. "Remember that everyone is different!" the 32-year-old wrote in the caption to a photo of her adorable bump at 13 weeks.

Skye is absolutely right, of course. "Women come in all different shapes and sizes, and carry differently depending on a number of factors," says Jessica Kiley, MD, associate professor of obstetrics and gynecology at Northwestern Feinberg School of Medicine.

If this is your first pregnancy, for example, it may take a little longer for your uterus to move up and out of your pelvis. During subsequent pregnancies, "the abdominal wall has greater laxity, allowing women to show sooner," Dr. Kiley says.

Another factor that can affect the size of a woman's bump is a condition called diastasis recti, which is when the left and right sides of the rectus abdomens muscles separate. That produces more space in the wall of your abdomen, Dr. Kiley explains, and the uterus may push out more, making the bump appear bigger.

A woman's level of fitness and general abdominal strength may also play a role in the shape her belly. But one variable that doesn't influence the size of her bump is her body type.

During the second half of pregnancy, a woman's doctor will measure her fundal height, which is the length of her uterus from top to bottom. "Regardless of body type, women will tend to have a normal fundal measurement for how far along they are," says Dr. Kiley.

Dr. Kiley tells most of her patients they will likely start to show around 20 weeks, but may pop sooner or later, and that's okay. Try your best to resist comparing yourself to other moms-to-be. "Pregnant women always feel like they look bigger or smaller than their friend, sister, or neighbor," says Dr. Kiley. "It's our job as doctors to explain that what's normal for them might not appear normal for other women."