I was born in January, but I was a month early. My dh was born in October, and my kids were born in June, August and October (but the October baby was due in November, being a month early.)
As far as I know (working as a Maternal Infant Health nurse) the most common month for births is September. This is followed by August, June and July.
The problem is too many babies born recently were born due to artificial forms of labor induction and "planned" C Sections, so most births in the last 10-15 years do not go the full 40 wks. Of course, when you say "40 weeks" you are started the "pregnancy" on the FIRST day of a woman's last menstrual period, so technically, OB considers "charting the pregnancy" to start (logistically, not clinically) about 2 wks before the baby was even conceived. Medicine and science understand the difference between this timing and what actually, clinically determines "pregnancy." THAT means a fertilized egg implanted into the wall of the uterus. Clinically, "fertilization" is NOT considered pregnancy. I'll explain why later.
Taking into account it can take up to 10 days for a fertilized egg to implant in the uterus and then turn into a clinical "pregnancy." The TRUE definition of "pregnancy" is when a fertilized egg implanted in the uterine wall, NOT conception, which is the common and commonly naturally disposed of, meeting of sperm and egg.
A good half or more of all fertilized eggs are simply discarded
by the woman's body naturally
, as being judged genetically inferior for carrying. This natural method helps healthier babies being born. This natural method is an other reason why methods like IGF, IVF and GIFT have a MUCH higher rate of pregnancy and fetal health issues than natural conceptions. What ever egg ripens first and any sperm they can catch with the tip of the syringe are plopped together, not allowing Nature to make the decision on the strongest, best or highest quality cells, as happen with natural, untampered with conceptions. Although these methods may be the only way some couples can have children at all.
Years ago, when these methods were newer, most doctors would have a couple try for at least a year or two before suggesting external methods of assisted conception. Now, many doctors and clinics (who make MONEY from fertility "problems") will suggest IVF or even more complicated forms of assisted conception after only ONE cycle where the woman doesn't conceive naturally.
The average pregnancy today is less than the old standard of "40 weeks." It's more like 37-38 wks, although so many OBs simply schedule inductions and C Sections for when they are convenient, we have terrible problems with "immature" and premature babies being born.
So, birth date has really little to do with when the baby may have been implanted.
But, as it is, September is the month more babies are born than any other. That means Dec or January are the months most common for conception.... maybe...