Hearing your baby's heart for the first time is a magical moment. In some cases, that beat is joined by an echo – a sign that in just a few short months, there will be twice the cuddles, twice the diapers, and twice the patter of little feet.
On average, 12 out of every 1,000 deliveries around the world today are twins. The statistic is the highest seen in decades, and most likely the highest it's ever been.
But for all the joy multiple births bring to families, not all implications of increased chances of twins are cause for celebration.
Researchers from Radboud University in the Netherlands, the University of Oxford in the UK, and the French Institute for Demographic Studies dug through demographic databanks of 165 countries dated from 2010 to 2015, tallying numbers of twin deliveries.
Compared with similar records collected in the early 1980s, it seems we're popping out babies two at a time at a rate that's now 30 percent higher, jumping from 9.1 twin deliveries per 1,000 total, to 12 twins for every 1,000 deliveries.
Though the records studied in this particular investigation were limited to just the two relatively recent historical periods, the researchers behind the study are betting that it's a record throughout history.
"The relative and absolute numbers of twins in the world are higher than they have ever been since the mid-twentieth century, and this is likely to be an all-time high," says University of Oxford sociologist Christiaan Monden.
The analysis focused on changes in individual populations around the globe to create a detailed picture of reproduction patterns between demographics over time.
Just seven of the 112 nations for which 1980s data could be found saw a drop of more than 10 percent in the number of twins, most of which were in South America.
In a whopping 74 countries, the rise was above 10 percent, ranging from a 32 percent rise in Asia to a 71 percent jump in twinning rates in North America.
African nations also saw a significant rise in twins being born, from an average of 32 percent of all twin deliveries between 1980 and 1985 to 41 percent in the period of 2010 to 2015.
To what do we owe this relative explosion in multiple births? For the most part, it's a mix of two factors.
One is more parents are using medically assisted reproductive technologies, such as in-vitro fertilization. These procedures have tended to maximize odds of a successful birth by fertilizing and implanting multiple embryos, which carries a higher incidence of multiple births.
This is fortunately a trend that looks to be falling in coming years, as medical advances are gradually improving success rates so that only a single embryo needs to be implanted in any cycle.
The other factor is the fact that prospective parents are holding off on building families until later in life, which is also likely to be a major contributor to higher rates of twins.
A study similar to this conducted in the US several years ago found that beyond age 35, white women were three times more likely to have non-identical 'fraternal' twins. For black women, the figure was four times.
Modern medicine in more affluent corners of the globe means many of us have don't have to think as much about the risks that delivery brings for mother and child, especially for older women. It's easy to forget that each birth raises the odds of complications.
"This is important as twin deliveries are associated with higher death rates among babies and children and more complications for mothers and children during pregnancy, and during and after delivery," says Monden.
For many low-socioeconomic nations, the risks are only compounded further, making rising numbers of twins a serious concern even months after the twins are born.
Jeroen Smits, director of the Global Data Lab and professor of Economic and Human Development at Radboud University, points out: "In sub-Saharan Africa in particular, many twins will lose their co-twin in their first year of life, some two to three hundred thousand each year according to our earlier research."
The team intends to continue monitoring birthing rates in coming years to see if the trends of multiple births have seen their peak. Changes to how we build families are likely to see populations around the world shift in dramatic ways in coming decades.
By ensuring universal access to healthcare and advanced medicine, we can ensure hearing those two heart beats for the first will be music to all parents' ears.
This research was published in Human Reproduction.