You may have been shocked by reports of a 33-year-old California woman birthing octuplets in January—until, that is, there was news of a woman with two uteruses who popped two babies—one from each womb. The mother, dubbed "Womber Woman" by the New York Post, last week delivered two baby girls at Marquette General Hospital in Marquette, Mich., according to The Mining Journal. The twins were seven weeks premature and underweight—one weighed in at three pounds, 15 ounces (1.8 kilograms) and the other four pounds, 15 ounces (2.2 kilograms)—but doctors told the newspaper that both babies (and their 21-year-old mom) were doing just fine after their cesarean section delivery.

The woman, Sarah Reinfelder, reportedly was born with a condition called uterus didelphys, or double uterus, which affects a number of women who may not even be aware they have it. We asked Robert Zurawin, an obstetrician/gynecologist (ob–gyn) at Baylor College of Medicine in Houston to explain the condition and its implications for fertility.

[An edited transcript of the interview follows.]

How common is it to have a double uterus?
One in about every 2,000 women worldwide have the condition.

How likely is it that a woman with this condition would get pregnant with twins—one baby growing in each uterus?
About one in 25,000 women with uterus didelphys gets pregnant with twins, one to each uterus. [That means the likelihood of any given woman growing two babies in two separate wombs is about one in 50 million.]

How would you know if you have two uteruses?

Most women aren't even aware they have the condition until they become pregnant and get an ultrasound exam, a test that uses sound waves to produce images of the inside of the body. If she gets an ultrasound about eight weeks into her pregnancy (as most ob–gyns recommend), chances are the ultrasound technician would spot the extra womb. But if the woman doesn't get an ultrasound until 20 weeks or more into her pregnancy, the uterus housing the fetus might have grown big enough to overshadow the extra uterus in which case the ultrasound technician might not see it (and the woman may never know).

How does a woman end up with two wombs?
In the embryonic stage of human development, a female has more than one "uterine horn," or tubes that ultimately fuse into one uterus. In people with this condition, somewhere in the developmental process the tubes didn't come together, most likely because there was an error in the signals cells received instructing them to migrate to certain places.

Where do the double uteruses sit in the body, and what do they look like?
They are in the same place that a single uterus would be—in the pelvis right behind the bladder and in front of the rectum. Assuming the woman is not pregnant, the twin uteruses take up about as much space as a single uterus, which is about the size of a pear. Together they resemble a valentine's heart, each having a round top and a tapered bottom. (So if you cut the heart down the middle, each side would look like a uterus.)

If you have two uteruses, do you also have two cervices (the narrow end of the uterus, where a baby's head emerges during birth) as well as two vaginas?
Some people with uterus didelphys also have two cervices and two vaginas, but some only have one vagina. Most women with two vaginas do not get surgery to fuse them, because one side is typically bigger than the other, so they have intercourse using just that one side.

Does having two wombs make it easier or harder to get pregnant?
Normally, women with two uteruses don't have problems getting pregnant, but in some cases it might be harder. Women with uterus didelphys sometimes do not have a fully developed uterine lining, the layer of tissue covering the inner wall of the uterus, which provides a place for implantation of a fertilized egg and attachment of the placenta that supplies oxygen and nutrients to the growing fetus. In other cases, the uterus might have an irregular shape that prevents it from stretching to accommodate the growing fetus.

Why were the twins in Michigan born prematurely—before the normal 37 to 40 weeks of gestation?
About 60 percent of all twins are born prematurely. This probably has something to do with the fact that the uterus (or in this case, the double uteruses) can only expand so much. In some women with this condition, the two uteruses cannot expand as much as a normal, single uterus would.

When babies are born this way (coming from different wombs in the same mother), are they delivered at the same time?
Yes, both uteruses go into labor at the same time, so the babies would likely be born within minutes of one another.

Can a woman with this condition have babies less than nine months apart that are full-term; that is, can she have, say, a two-month-old fetus in one womb and conceive another in her second uterus?
You can't get pregnant in a second uterus when you are already pregnant in the first. The body would not support ovulation (the release of a mature, ready-to-be-fertilized egg from the ovary) when a pregnancy has already begun.

Do other animals have two uteruses?
A lot of mammals, including pigs, dogs, rabbits and cats have two uteruses. All primates have single uteruses. In these other animals, multiple fetuses can grow in each uterus; the fetuses share the placenta, but each one has its own umbilical cord.