Starting in your 30s, especially later 30s makes this extremely difficult. It takes months of perfectly hitting ovulation windows to get pregnant, and then you generally want at least ~9 months between children, so you're looking at ~2 years per child, all while fertility starts to rapidly decline as you head into your 40s.
The long and short is that a sustainable population is going to require the majority to start having children in their 20s. That figure going down has nothing to do with teen pregnancy.
30% of couples who don’t use birth control and who have regular sex get pregnant within one month.
60% get pregnant within three months.
80% get pregnant within six months.
"Women younger than 30 have about a 20 percent chance of getting pregnant naturally each month. By age 40, the chance of pregnancy is about five percent each month." [1]
Things like IVF do not dramatically change the odds either. They're better of course, but it's far from guaranteed - it's still just a rather expensive roll of the dice.
Then on top of all of this, having children later greatly increases the chances of miscarriage, developmental issues (like Down syndrome) and so on.
Life's brutal here - you're in a race against time, yet the later you start the longer it takes, and the harder it becomes.
> Things like IVF do not dramatically change the odds either. They're better of course, but it's far from guaranteed - it's still just a rather expensive roll of the dice.
Sadly, literally all of our peers who were trying and having kids in the same demographic as us, +/- a few years, struggled hard and most needed fertility treatments. At least 2-3 of them were never able to conceive, despite the expensive and time-consuming treatments. It's brutal.
I will also add, I have a number of friends who are >= 10 years younger than I am and many of them also struggled with miscarriages in their late 20s while trying to start families.
These are extremely important years for career building.
It's more about the fact that young women are choose to build their education and career (like men do) rather than have children.
In most countries with falling birthrates (save some exceptions like Japan) a huge part of this decrease is in the 15-25 age group. There is usually also a decline in the 25-29 but there's an increase or the same rates as we've had historically on the 30+ range.
What this mean is basically that we've eradicated teen pregnancy, and that's definitely something desirable in a developed society.