A British woman plans to have a baby for the second time.
The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their second child
— Clarence House (@ClarenceHouse) September 8, 2014
It is not yet clear whether Prince George, 13 months old, has been notified. What is evident, though, is that George will be younger than most British first-borns who become siblings.
Since the former Kate Middleton’s due date has not yet been announced, we can only estimate that the interval between royal births will be somewhere between 20 and 22 months. That’s considerably shorter than the British median of 36 months between a first and second child.
According to the U.K.’s Office for National Statistics, that period is longer between a second and third child (41 months) but similar to the gap between a third and fourth child (37 months). None of those figures has changed much over the past 10 years.
In the United States, the national data is presented slightly differently — rather than give the median number of months between first and second birth, the Centers for Disease Control and Prevention (CDC) publishes a percentage breakdown of different birth intervals. There is a gap of 13 to 24 months between first and second child for 17.5 percent of Americans and a gap of 25 to 36 months for 17.2 percent of Americans. The CDC bases its calculations on the National Survey of Family Growth, which interviewed 22,682 men and women between 2006 and 2010.
These figures don’t indicate how long parents “wait” to have baby No. 2 — childbirth isn’t so straightforward. Births that are registered in the National Survey of Family Growth are classified as either “intended” or “unintended.” Of those that fall into the latter category, the CDC specifies whether those births are “mistimed” or “unwanted.”
From 2006 to 2010, 23.3 percent of all American births were “mistimed.” More specifically, parents said 9.2 percent of births were less than two years too soon and 14 percent were two or more years earlier than they had hoped (presumably the former category includes births that were later than the parents had wanted). The likelihood that a birth will be described as “mistimed” rises for younger mothers, for those who are neither married nor cohabiting, and for first births.
Mistimed births are clearly not a U.S.-specific phenomenon. But the difference between actual birth interval and preferred birth interval varies considerably by country.
In 2011, USAID published a report that looked at trends in birth spacing in 72 developing countries. The data comes from the Demographic and Health Surveys, which were conducted from 1985 to 2008, so there’s variance in how up-to-date the information is. I excluded those countries for which the surveys were conducted before 2000.
Of the 51 remaining countries, the median birth interval between all children (not just first and second) was 32.5 months. But the desired interval — how long women said they would like to wait before having another child — was 38.3 months.
In almost every country the researchers looked at, women gave birth to an additional child before they would have preferred. The gap between actual and preferred birth intervals was largest in Peru (23.4 months) and smallest in Nigeria (0.5 months). Only in three countries did women report having birth intervals that were longer than they would prefer.