Researchers studied 270,000 pregnancies between 2004 and 2009 and found babies conceived in December resulted in the highest birth rates.
There were three extra babies surviving per every 200 pregnancies compared with those conceived in the summer months, according to the researchers at Indiana University.
Offering a explanation, Dr Paul Winchester, who led the research team, said mothers who conceive in December or January receive high levels of sunshine at a late stage of their pregnancy.
This raises the level of vitamin D they get, which promotes a healthy birth.
Dr Winchester, who was speaking at the annual American Society for Reproductive Medicine meeting, said high levels of pesticides sprayed on crops during the summer may play a part in the “toxic June effect”.
He said mothers who conceive in June at at their "most vulnerable stage" and they were more likely to have shorter pregnancies and premature babies.
He added, according to the Telegraph: "There are a lot of things we are finding that are seasonal and very disturbing. We have seen significant seasonal differences in reproduction.
"Valentine’s Day is one of the least likely times to conceive a baby, whereas Christmas seems a very positive time."
Backing up this theory, research published by Cambridge University found that babies born in the summer months were likely to be stronger and taller.
They also said this could be because summer pregnant mothers absorb more Vitamin D from sunshine in the latter parts of their pregnancy, which is important for maintaining healthy bones and is thought to protect against cancer, type 1 diabetes and multiple sclerosis.
To have a summer pregnancy, women will have to conceive around December/January.