It is not because women are older
The rise in severe complications tracks blood pressure and weight far more than it tracks age.
The story you have heard is that American pregnancy is getting riskier because American women are having babies later. Age has risen. The rest of that sentence does not survive contact with the data.
Age has risen, by about a year. The mean age of a woman giving birth went from 28.7 in 2016 to 29.7 in 2024. Mean age at a first birth went from 26.6 to 27.6, a record high. In 2024, 774,908 births were to women aged 35 and over, about 21.4% of all births. Both of those figures are arithmetic done here on the published age-group counts rather than numbers NCHS prints as such.
Now the decomposition. A 2026 analysis of more than 25 million hospital deliveries held each risk factor at its 2016 level in turn and asked how much of the rise in severe maternal morbidity each one explained. The finding, in the authors' words: HDP accounted for 32.4% (95% CI, 31.2-33.7%) of the increase in SMM and 51.3% (95% CI, 48.5-54.9%) of the increase in nontransfusion SMM. Chronic hypertension and obesity also contributed meaningfully, whereas advanced maternal age and gestational diabetes accounted for a smaller share of the increase.
High blood pressure disease explains about a third of the rise, and about half of the rise in the stricter measure. Age explains less.
Some things genuinely are age-graded, and here they are in absolute terms. Per 1,000 live births in 2024, gestational diabetes went from 25.3 under age 20 to 152.9 at 40 and over, and chronic high blood pressure from 10.6 to 69.6. Per 100,000 live births, admission to intensive care went from about 160 at ages 25 to 29 to about 406 at 40 and over, and unplanned hysterectomy from 8.7 under 20 to 121.2 at 40 and over. Those are real increases. They are also increases between two small numbers, which is why they should always be given this way rather than as "doubles after 40."
Not everything runs one way. Infection in labor falls with age: 26.1 per 1,000 in women under 20, 13.1 per 1,000 at 40 and over. Induction is lowest in women aged 30 to 39.
Two age effects are steep and should be said plainly. ACOG's 2018 guidance on early pregnancy loss reports that the frequency of clinically recognized early pregnancy loss for women aged 20-30 years is 9-17%, and this rate increases sharply from 20% at age 35 years to 40% at age 40 years and 80% at age 45 years.
And stillbirth risk is not a straight line with age but a U. ACOG reports a rate of 15.88 per 1,000 live births for girls under 15, nearly three times the rate in the lowest risk group, women aged 25 to 29, at 5.34 per 1,000 live births. Read those two next to the national stillbirth figures elsewhere in this guide with care: NCHS counts stillbirths per 1,000 live births plus fetal deaths, and ACOG's sentence here uses live births alone as the bottom. The two are close in size and are not the same measure.
"Which of my risks come from my age, and which come from something we could actually work on?"
"Give me the absolute numbers, not the multiples."