At how many weeks does a woman start showing

• You'll probably start to show in pregnancy at 12 to 16 weeks. 
• You're more likely to show early if you're short, carrying multiple babies, or you've been pregnant before. 
• By week 20, your doctor will start measuring your fundal length, the distance between the pubic bone and the top of the uterus. Typically, this number should correlate with your week of pregnancy.
• This article was reviewed by Mahino A. Talib, MD, who is a clinical assistant professor with the Department of Obstetrics and Gynecology at NYU Langone.
• Visit Insider's homepage for more stories.

Pregnancy bumps are as diverse as the babies they produce. 

You start to show in pregnancy at 12 to 16 weeks 

If it's your first pregnancy you'll probably start to show between 12 to 16 weeks. But this initial bump is not from the baby. In fact, at 15 weeks, the average fetus is 4 inches long, or about as large as an orange. 

So that "baby bump" isn't from the baby's size, but rather the expansion of your uterus.

"As the uterus gets bigger with a developing pregnancy, the loops of bowel which fill the abdomen are pushed upwards and out to the sides," says Meg Wilson, MBBS, an OB-GYN at London Gynaecology.

When you'll probably show early

• Height: If you have a short abdomen, your bump will likely pop out sooner than for taller women with a longer abdomen. "Women who have a long abdomen may have more space for their uterus to develop upwards rather than outwards, which can give the appearance of a smaller bump," Wilson says.  
• Weight: If you're smaller with less body fat, you'll probably show much earlier than someone with more body fat. 
• Multiple buns in the oven: If you're carrying twins or multiples, you're more likely to show earlier — as early as 6 weeks. That's because your uterus will expand more to create space for the additional fetuses. 
• Not your first rodeo: You're more likely to show early if this isn't your first pregnancy.  "The uterus is held in position at the front by the abdominal walls: skin and muscle strength," says Wilson. "For women who have had several pregnancies, the skin and muscle become weaker and the pregnant uterus can fall forward more."

When you should worry about bump size

By week 20, your doctor will start measuring your fundal length — that's the distance between the pubic bone and the top of the uterus. Typically, this number should correlate with your week of pregnancy.

So, for example, if your fundal length is 25 centimeters, you should be in or near your 25th week of pregnancy. 

If, however, your fundal length is too high, it could indicate growth problems with the baby like fetal macrosomia, which is when a baby is significantly too large. 

On the flip side, if your fundal length is too low it could mean restricted growth, which is when the baby isn't large enough. While not all small babies suffer from restricted growth, it's worth checking in with your maternal health practitioner if you are concerned. 

Doctors caution, though, that the fundal length isn't a perfect measurement, and variables like weight or carrying multiples babies can change your numbers significantly. 

Related stories about pregnancy:

• How much weight you should gain during pregnancy, according to doctors
• How to increase your breast milk supply right after giving birth
• How long it takes to recover from a C-section before you can drive a car, take a bath, and have sex
• Yes, Tylenol is safe for pregnancy. Here's the right dosage
• Foods to avoid during pregnancy and how to still stay healthy
• Doctors debunk the 25 biggest pregnancy myths

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