A pregnancy is considered high risk when a health condition, age factor, or lifestyle issue raises the chances of complications for the mother, the baby, or both. Some of these factors exist before conception, while others develop during pregnancy itself. Understanding what puts a pregnancy into this category can help you recognize risks early and get the right level of care.
Maternal Age at Both Extremes
Age is one of the most well-documented risk factors. Pregnancies in people 35 and older carry progressively higher rates of nearly every major complication. Miscarriage rates climb steeply with age: roughly 12% for those under 30, 25% for ages 35 to 39, 51% for ages 40 to 44, and 93% for those 45 and older. The risk of ectopic pregnancy (where the embryo implants outside the uterus) increases four- to eight-fold after 35.
Preeclampsia, a dangerous blood pressure condition, affects 3% to 4% of pregnancies overall but jumps to 5% to 10% in those over 40 and as high as 35% in those over 50. Stillbirth risk at full term also rises considerably: about 3.7 per 1,000 ongoing pregnancies for first-time mothers under 35, compared to 8.65 per 1,000 for those over 40. Cesarean delivery rates follow the same pattern, going from about 20% in the 25-to-34 age group up to 61% for those 50 and older. Postpartum depression is also significantly more common, with nearly four times the odds in mothers aged 40 to 44 compared to those in their early thirties.
On the younger end, teenage pregnancies carry their own set of risks. Adolescent mothers are more likely to be anemic, underweight, and less likely to receive adequate prenatal care. The combined rate of spontaneous miscarriage and stillbirth in teenage pregnancies is about 9.8%, compared to roughly 7% in the general population. Preterm delivery and low birth weight are more common, partly because adolescent bodies are still growing and competing with the fetus for nutrients.
Pre-existing Diabetes
Diabetes that exists before pregnancy (type 1 or type 2) creates risks that depend heavily on blood sugar control, especially in the earliest weeks. High blood sugar acts as a direct toxin to a developing embryo. It can cause heart defects, spinal cord abnormalities like spina bifida, skeletal malformations, and miscarriage, all in a dose-dependent way: the higher the blood sugar, the greater the risk.
When blood sugar is well controlled before conception, with an A1c at or below 6%, the risk of birth defects drops to 1% to 3%, which is about the same as the general population. That’s why medical guidelines recommend getting A1c below 6.5% before trying to conceive, and ideally below 6% during pregnancy. Later in pregnancy, poorly controlled blood sugar causes the baby to grow excessively large, particularly around the abdomen and shoulders. This increases the chance of a difficult delivery, shoulder injuries during birth, and cesarean section.
Chronic High Blood Pressure
High blood pressure that predates pregnancy raises the risk of preeclampsia, placental abruption (where the placenta separates from the uterine wall), preterm birth, and fetal death. A large trial of more than 2,400 pregnant women with chronic hypertension found that when blood pressure was treated at a threshold of 140/90, the rate of serious complications dropped from 37% to about 30% compared to waiting until blood pressure reached severely elevated levels. Based on this, current guidelines recommend treating blood pressure at 140/90 during pregnancy rather than the previously used, more lenient threshold of 160/110.
Blood pressure that spikes in the second half of pregnancy also requires close monitoring because it can signal the development of preeclampsia on top of existing hypertension, a combination that carries particularly serious risks.
Obesity and Underweight
A BMI of 30 or higher at the start of pregnancy places it in a higher risk category. The risks increase with each step up in BMI. Compared to normal-weight individuals, those with severe obesity (BMI of 40 or above) face roughly three times the odds of birth defects, nearly four times the odds of having an unusually large baby, and about 2.8 times the risk of stillbirth. Babies born to mothers with severe obesity are nearly three times more likely to need intensive care after birth and more than three times as likely to die in the first week of life.
Heart defects, neural tube defects, and other structural abnormalities all occur at higher rates. At BMIs of 35 and above, premature birth before 34 weeks is about twice as likely. Being significantly underweight (BMI below 18.5) also carries risks, including low birth weight and preterm delivery, though the research base is smaller.
Placental Problems
The placenta can cause life-threatening complications when it attaches in the wrong place or grows too deeply into the uterine wall. Placenta accreta spectrum is a condition where the placenta invades the uterine muscle instead of sitting on top of it. The biggest risk factor is a prior cesarean delivery, and the risk increases with each additional cesarean. This condition can cause massive hemorrhage during delivery and frequently requires removal of the uterus.
The rising rate of placenta accreta over the past four decades tracks closely with the increase in cesarean deliveries. A scar from a previous cesarean creates a weak spot in the uterine lining where the placenta can anchor abnormally deep. Placenta previa, where the placenta covers or sits near the cervix, is another complication that causes bleeding and often requires early delivery. Detecting these conditions before labor through ultrasound imaging significantly improves outcomes.
Multiple Pregnancies
Carrying twins, triplets, or more multiplies nearly every pregnancy risk. More than 3 in 5 twin pregnancies end in preterm birth (before 37 weeks), and that rate is even higher for triplets and beyond. Preterm birth is the single biggest threat in multiple gestations because it drives most of the complications babies face, from breathing problems to long-term developmental issues.
Identical twins who share a placenta face a unique risk called twin-to-twin transfusion syndrome, which occurs in about 15% of these pregnancies. In this condition, blood flow between the twins becomes unbalanced through shared blood vessels in the placenta, causing one twin to receive too much blood and the other too little. Without treatment, it can be fatal for both babies. Multiple pregnancies also raise the mother’s risk of gestational diabetes, preeclampsia, and cesarean delivery.
Sexually Transmitted and Other Infections
Untreated infections during pregnancy can harm both mother and baby. Syphilis is one of the most dangerous: more than half of pregnancies in women with untreated syphilis result in adverse outcomes, and congenital syphilis rates have surged 87% worldwide over the past five years. About half of untreated chlamydia and gonorrhea infections pass to the baby during birth. Gonorrhea can cause eye infections that lead to blindness if untreated, and chlamydia causes pneumonia in 10% to 20% of exposed newborns.
Gonorrhea infections also increase the risk of preterm birth, premature rupture of membranes, and low birth weight. Herpes simplex virus affects 1 in 3,000 to 1 in 10,000 pregnancies, and while most neonatal herpes cases involve the skin, eyes, or mouth, 1 in 4 affected newborns develops a life-threatening infection that spreads through the body. Trichomonas, a common and often overlooked infection, is associated with preterm delivery and low birth weight. Collectively, infections account for 7% to 30% of the roughly 2.6 million stillbirths that occur worldwide each year.
Smoking, Alcohol, and Drug Use
Smoking during pregnancy is linked to miscarriage, ectopic pregnancy, placental abruption, placenta previa, restricted fetal growth, low birth weight, stillbirth, and preterm delivery. Nicotine interferes with brain development and is associated with a higher rate of facial clefts and other structural abnormalities. Even after birth, prenatal nicotine exposure affects the parts of the brain that control autonomic functions like breathing.
Alcohol is toxic to a developing embryo at every stage. Heavy use increases the risk of miscarriage, stillbirth, low birth weight, and preterm delivery. Even low levels of alcohol exposure affect brain development in animal studies. The long-term effects on children include growth problems, behavioral issues, and deficits in thinking and motor skills.
Cannabis use is associated with low birth weight, smaller head circumference, preterm delivery, stillbirth, and later problems with executive function, depression, and behavior. Opioid use raises the risk of restricted growth, preeclampsia, placental abruption, preterm labor, and stillbirth, with children facing higher rates of sudden infant death syndrome and long-term cognitive problems. Cocaine use increases the chances of preeclampsia, placental abruption, preterm birth, low birth weight, and fetal death.
Previous Pregnancy Complications
Your history in prior pregnancies is one of the strongest predictors of risk in future ones. Preeclampsia recurs in roughly 10% to 17% of subsequent pregnancies, with one study of nearly 2,900 women finding that 17.1% developed it again. Prior preterm birth, gestational diabetes, and placental problems all increase the likelihood of the same complication happening again. A previous cesarean delivery, as noted, specifically raises the risk of placenta accreta in future pregnancies.
How High-Risk Pregnancies Are Monitored
When a pregnancy is classified as high risk, monitoring becomes more frequent and more detailed than standard prenatal care. Non-stress tests, which track the baby’s heart rate in response to its own movements, are one of the most common tools. Specialized ultrasounds at key points, particularly around 32 weeks, can detect babies that are growing too slowly. A targeted ultrasound at that stage has been associated with a four-fold reduction in stillbirths and newborn deaths among high-risk groups.
Doppler ultrasound, which measures blood flow through the placenta’s blood vessels, helps assess whether the placenta is functioning well. Continuous heart rate monitoring during labor is particularly important: hospitals that use it see dramatically lower rates of stillbirth during delivery compared to those that don’t. For some conditions, home monitoring of fetal movement using wearable devices is being explored, though the technology is still being refined. The overall goal is catching problems early enough to intervene, whether that means adjusting medication, timing delivery earlier, or transferring care to a higher-level facility.