In the Q&A section of The New York Times science section, a reader asked C. CLAIBORNE RAY, "Can a woman have two fetuses indifferent stages of development in the uterus at the same time?"
The short answer is: yes.
As for the long answer, the phenomenon is known as “superfetation.”
As Time explains, the phenomenon occurs when a woman continues to ovulate after becoming pregnant. This allows a second egg to fertilise and implant itself in the the womb.
"Typically, hormonal changes prevent further ovulation and thicken the lining of the uterus to preclude a second embryo from attaching."
Before you panic, it’s very rare.
A French report of one apparent case, published in 2008 in The European Journal of Obstetrics & Gynecology and Reproductive Biology, said each fetus had a separate amniotic sac and the difference is size was obvious throughout the pregnancy.
The report added that there were fewer than ten cases known.
The most recent of the cases was in 2006 when two children were conceived separately, two weeks apart. They were born healthy via C-section.