A few take away points:
As the Washington Post article makes clear large driving force behind this "declining birthrate" is that teenaged and young women are having fewer unplanned pregnancies. Be this via better access to birth control and or abortion, along with other factors this undoubtedly is a good thing. It was nor never has been a good thing for "babies to have babies".
Article also points out something many of us already know; women are choosing to delay having children well into the their late 30's or even 40's. That demographic is having babies, but just not enough of them and or fast enough to negate the decline from teenage pregnancies.
There are several factors at work here; access to birth control and or abortion has reduced teenaged pregnancy; while advances in fertility mean women can (or so they are told) put off having children well into middle age.
Here in NYC am here to tell you what once was a rarity (twins, triplets and other multiple births) has become common place for years now. Why? So many women waiting until they are in their late 30's or well into their 40's to have children.
What can and often does happen is after two or more decades of being on birth control what they are lead to believe (simply stopping will allow them to become fertile and conceive easily) often just does not happen. After a few years of "nothing" women or couples head off to a fertility doctor and being various treatments. IVF and a few others are well known to often result in multiple births. Hence all these twins and so forth.
Have no proof (obviously), but Beyoncé at just gave birth to twins. She isn't *that* old (only 35) but again women in their thirties can and often do have problems conceiving naturally.
Tying into all this is the explosive growth and acceptance of surrogacy. Which also explains why so many parents both gay and straight have twins or other multiples.
Bottom line in all of this the same thing seen all over the western/developed world. Women are gaining more education and independence, as such are either choosing to forego having children and or putting it off until later in life. The idea so common in the 1950's right through say until the 1980's or so of going to college then dropping out of the workforce after a few years to have babies and stay home seems antediluvian to young women today.
The increased acceptance of homosexuality is going to affect birthrates as well. Even though gay males represent statistically small part of overall demographics, a man or two men cannot have children on their own. So unless they can afford a surrogate (and have access) they won't be having any children.
Other major issue is that overall the United States policies towards children largely remains stuck in another century. That is it is basically up to the parent or parents to provide/deal with things.
Lone among western developed nations the USA does not have any sort of mandated paid maternity/parent leave on the federal level. Maternity, post natal and baby/children child care is expensive and access to quality care varies by many factors.
USA lone also among developed nations does not offer any sort of federal mandated/funded childcare/crèche.
In contrast look at France, a country that has been dealing with declining birth rates off and on for over one hundred years or more:
Having a baby in France and maternity leave in France | Healthcare | Expatica France
Catching Up With France on Day Care - The New York Times