More people are having children in their 30s and above, but what are the risks that older parenthood presents?
Recent preliminary data from the Centers for Disease Control and Prevention (CDC) indicate that in 2016, for the first time in three decades, birth rates among women aged 30 to 34 surpassed those among women aged 25 to 29 in the United States.
The average age that women are having their first child is currently at around 28 years of age, up from 26.4 in 2015, and 26.3 in 2014. Experts often focus on average or mean maternal age due to the birth outcomes that are associated with the age of the mother, such as multiple births and congenital disabilities.
Studies report that people may delay parenthood until 35 or older for a number of reasons.
These reasons include women reaching higher education levels, establishing their career, improved methods of contraception, social and cultural shifts that have left women feeling not ready to have children, lack of childcare, low benefit levels, inflexible workplace policies, economic or housing uncertainty, and unemployment.
Another reason why women are going through pregnancy later in life could be down to improved fertility options, such as IVF.
Pregnant women over the age of 35 and having their first baby have been termed as being advanced maternal age (AMA) or older mothers, or they are being referred to as an elderly primigravida or elderly primipara. The terms "advanced age" and "elderly" have negative connotations for someone of just 35 years. Are these terms unfounded, or does being over 35 pose a serious risk for the mother and baby?
Risks of delaying pregnancy until age 35 years or older
Fertility declines in both men and women with age.
Everyone is aware of the ticking of the biological clock, but does your 35th birthday represent a particularly special milestone in biology? Do you hit 35 and suddenly become "high risk" overnight?
Women are delivering healthy babies throughout their 30s and beyond. The age of 35 is simply an age that certain risks become more worthy of discussion.
While these risks become slightly more likely after hitting 35 years old, this does not mean that they will have a significant impact on everyone in their mid-thirties and older.
Decline in fertility
Women are born with all the eggs they will ever have. As females age, the likelihood that they will get pregnant reduces due to the declining number of remaining eggs and their reduced quality.
Fertility also declines in men with age due to declining sperm counts, motility, and semen volume. These age-related factors combined can make it more difficult for women to become pregnant.
One study, published in The New England Journal of Medicine, found that among women who received artificial insemination, 74 percent of those under 31 years old were pregnant within a year. However, this decreased to 61 percent of individuals between the ages of 31 to 34, and it further declined to 54 percent of women aged 35 and over.
Certain genetic risks present more often in pregnancy as women age. For example, the rate of having a baby with Down syndrome accelerates with maternal age.
While the rate of an embryo having Down syndrome at the 10-week mark of pregnancy is 1 in 1,064 at age 25, this rises to 1 in 686 at age 30 and 1 in 240 by the age of 35 years. At the age of 40, the Down syndrome rate increases still to 1 in 53, and down to 1 in 19 embryos at age 45.
A study published in Nature Communications set out to investigate why older mothers have a heightened risk of giving birth to children with congenital anomalies that are characterized by abnormal chromosome numbers.
Researchers from the Albert Einstein College of Medicine of Yeshiva University in New York learned that the genetic process of recombination could be responsible for the increased risk of conditions such as Down syndrome.
Recombination is the process in which pairs of chromosomes exchange genetic material before separating. The team found that in older mothers, the process of recombination may be less regulated, which may lead to abnormal chromosome numbers in sex cells or large chromosomal rearrangements.
The risk of miscarriage climbs gradually with the mother's age. Research published in the BMJ showed that risk of miscarriage is around 8.9 percent for women aged 20 to 24 years and increases to 74.7 percent for individuals aged 45 years or above. The declining quality of women's eggs is thought to be responsible for the higher rates of miscarriage.
Stillbirth is more likely in older women than younger women. A systematic review published in the Canadian Medical Association Journal found that stillbirth is around 1.2 to 2.23 times higher in older women.
Another study, examining data from 385,120 pregnancies in the United Kingdom, observed that the rate of stillbirth was 4.7 per 1,000 for women aged 18 to 34, 6.1 per 1,000 between the ages of 35 and 40 years, and 8.1 per 1,000 for women aged 40 and over.
Furthermore, the stillbirth rate has been shown to be higher in people having their first child and even higher in first-time moms aged 35 or older.
Women aged 35 years and older are often recommended to be induced as they approach their due date because of the increasing risk of stillbirth with gestational age. Around 1 in 1,000 women under 35 years old have a stillbirth during 39 and 40 weeks of gestation, compared with 1.4 in 1,000 women aged 35 to 39, and 2 in 1,000 women at age 40 and above.
The reasons that stillbirth rates increase with maternal age are currently unclear.
Research comparing pregnancy complications among women aged 18 to 34 years, 35 to 40 years, and 40 and over, found small increases in most pregnancy- and birth-related complications with age.
Birthing outcomes such as emergency cesarean delivery and postpartum hemorrhage are increased with maternal age.
The researchers identified increases in the risk of gestational diabetes, placenta previa, breech positioning of the baby, emergency cesarean delivery, postpartum hemorrhage, preterm birth, low birth weight, and high birth weight. Other research has found that risk of maternal mortality also increases with age.
Research presented at the American Stroke Association's International Stroke Conference 2016 revealed that compared with women who go through pregnancy at a younger age, pregnant women aged 40 and older are at greater risk of ischemic stroke, hemorrhagic stroke, heart attack, and death from cardiovascular disease.
"We already knew that older women were more likely than younger women to experience health problems during their pregnancy," said Dr. Adnan I. Qureshi, director of the Zeenat Qureshi Stroke Institute in St. Cloud, MN. "Now, we know that the consequences of that later pregnancy stretch years into the future."
Dr. Qureshi and colleagues found that all the risks, except for hemorrhagic stroke, were explained by well-known risk factors for cardiovascular disease - such as high blood pressure, diabetes, and high cholesterol - that older pregnant women face.
Why do women over age 35 have health, birth complications?
Recent research published in The Journal of Physiology sheds light on why women over 35 years old are more likely to face birth complications. Researchers from King's College London in the U.K. discovered, in a mouse model, that a mother's age influences the structure of the uterus.
In a group of mice that represented women over 35 years old, muscle contraction properties in the uterus were found to be impaired, less sensitive to oxytocin, and have reduced mitochondria numbers, all of which suggest that the uterus muscles are less able to contract properly. Furthermore, researchers found changes in progesterone signaling, which triggered a delay in labor.
"Our research highlights that there are key physiological and cellular changes associated with a mother's age that result in labor dysfunction," explained Dr. Rachel M. Tribe, lead investigator of the study at King's College London. "Timing of delivery and progress of labor is directly related to maternal age, and this can cause complications during birth."
Advanced age not responsible for low birth weight, preterm birth
Babies who are born at a low birth weight often have more respiratory, cognitive, and neurological problems, while preterm babies are more likely to have heart defects, brain damage, lung disorders, and delayed development.
A mother's age is not responsible for an elevated risk of low birth weight and preterm birth, according to researchers.
Some studies have indicated that there is an elevated risk of low birth weight (under 2.5 kilograms) and preterm birth (before 37 weeks of pregnancy) for women aged 35 or older. However, a recent study - conducted by Mikko Myrskylä, director of the Max Planck Institute for Demographic Research, and Alice Goisis, from the London School of Economics in the U.K. - suggests that a mother's age is not the reason.
The research, published in the American Journal of Epidemiology, found that the reasons are more likely to be associated with individual circumstances or behaviors that are more common in older adults.
Potential candidates for these factors could be fertility problems, which are connected with poor birth outcomes, maternal stress, and unhealthy behaviors.
"Our findings suggest that women should not be concerned about their age per se when considering to have a child," said Myrskylä. "It seems that individual life circumstances and behavioral choices are more important than age."
People deciding whether to postpone parenthood should take into account declining fertility and increases in the risk of miscarriage and birth complications. Stillbirths are relatively rare, but the risk remains higher for older women than their younger counterparts and increases in the final weeks of pregnancy.
There are, of course, many plus points of putting off motherhood until after age 35. Children of older mothers have been found to have fewer behavioral, social, and emotional difficulties.
According to another study conducted by Myrskylä and his colleague Kieron Barclay, at the London School of Economics, being born later is also associated with being healthier, taller, and obtaining more education.
"The benefits associated with being born in a later year outweigh the individual risk factors arising from being born to an older mother. We need to develop a different perspective on advanced maternal age. Expectant parents are typically well aware of the risks associated with late pregnancy, but they are less aware of the positive effects."
Despite the risks, most women aged 35 and older will have a normal pregnancy with few birth complications, and go on to deliver a healthy baby.