They have injected human stem cells into pig embryos to produce human-pig embryos known as chimeras.
The embryos are part of research aimed at overcoming the worldwide shortage of transplant organs.
The team from University of California, Davis says they should look and behave like normal pigs except that one organ will be composed of human cells.
The human-pig chimeric embryos are being allowed to develop in the sows for 28 days before the pregnancies are terminated and the tissue removed for analysis.
Creating the chimeric embryos takes two stages. First, a technique known as CRISPR gene editing is used to remove DNA from a newly fertilised pig embryo that would enable the resulting foetus to grow a pancreas.
The team at UC Davis hopes the human stem cells will take advantage of the genetic niche in the pig embryo and the resulting foetus will grow a human pancreas.
But the work is controversial. Last year, the main US medical research agency, the National Institutes of Health, imposed a moratorium on funding such experiments.
The main concern is that the human cells might migrate to the developing pig's brain and make it, in some way, more human.
Other teams in the United States have created human-pig chimeric embryos but none has allowed the foetuses to be born.
In the mid-90s there were hopes that genetically modified pigs might provide an endless supply of organs for patients, and that cross-species transplants were not far off.
But clinical trials stalled because of fears that humans might be infected with animal viruses.
Last year, a team at Harvard Medical School used CRISPR gene editing to remove more than 60 copies of a pig retrovirus.
Organisations are however campaigning for an end to factory farming; are dismayed at the thought of organ farms.
In Greek mythology, chimeras were fire-breathing monsters composed of several animals - part lion, goat and snake. The scientific teams believe human-pig chimeras should look and behave like normal pigs except that one organ will be composed of human cells.
Seven thousand people in the UK, for example, are on the transplant waiting list and hundreds die each year before a donor can be found.