The first time I got an epidural, it was too late.

I’d heard it was best to wait, for fear the medication would run out mid-labor (I later found out this is a myth). So I gritted my teeth through hours of contractions, and when I finally told the nurses I was ready, the anesthesiologist was with another patient.

Another unbearable hour passed before I finally got the medication. But by then, I was minutes away from pushing. I went through the final stage of labor feeling almost everything.

When I had my second baby, I got my epidural right away, but it made me incredibly itchy. Throughout 24 hours of labor, I scratched and wiggled so much that the epidural catheter, which administers the numbing medication, fell out, was put back in, fell out again, and wore off by the time I was ready to deliver.

During my third birth, the epidural provided a welcome numbing sensation on one side of my abdomen, but I still had sharp, intense pain on the other.

After this experience, I was frustrated. Were my anesthesiologists doing something wrong? Was I?

I’ve talked to dozens of mom friends over the years. Most say their epidural was like a magic wand that eliminated their pain in minutes. Some recalled napping or watching a movie through most of labor. I was envious and dumbfounded.

But epidural failure isn’t necessarily the fault of the medical provider or the patient.

“Epidurals are complex; it’s not like when you order something from a vending machine, it falls down and that’s what you get,” says Dr John W Patton III, director of regional anesthesia at Ronald Reagan Medical Center at UCLA. “Every patient is different, and a patient could have a different experience each time they receive an epidural, for various reasons.”

Here’s what you should know before getting an epidural – and why it might not work as expected.

What is an epidural?

An epidural is a form of regional anesthesia used for surgeries, traumatic injuries, chronic pain relief and childbirth. A thin, flexible tube called a catheter is inserted into the epidural space, a narrow corridor just outside the spinal cord. The catheter connects to a bag containing pain-relieving medication, administered through the tube to numb the connecting nerves. This continuously provides pain relief to the abdomen, pelvic region, lower back, and sometimes legs.

The main ingredient in epidural bags is the local anaesthetic, says Julie Steele, a certified registered nurse anesthetist and assistant clinical professor in the Nurse Anesthesia Program at Northeastern University; common ones are bupivacaine, chloroprocaine, lidocaine and ropivacaine. Some hospitals add low-dose opioids, such as fentanyl or morphine; others add a small amount of steroid, which can also help reduce pain.

Decades of research show that epidurals are one of the safest forms of pain medication for labor. “Studies have consistently found no long-term effects on babies’ health, development or bonding, and no increased risk of birth defects, autism or developmental issues,” says Dr Ashraf Habib, chief of the women’s anesthesiology division at Duke University. “Furthermore, epidural analgesia has an excellent safety profile for the mother, with serious complications being very rare.”

However, experts say they can sometimes decrease a patient’s blood pressure, which, if not treated, could put the fetus’s health at risk. In rare cases, an epidural can cause postdural puncture headaches, which are treatable but uncomfortable.

Why might a labor epidural not work?

There’s a chance an epidural may provide only partial relief, or in very rare cases, none at all.

“Epidurals aren’t foolproof,” says Steele. In her experience, epidurals don’t work as expected about 10% of the time, though studies report epidural failure rates range from 8% to 23%.

“You can’t get a 100% success rate with any medical procedure, including epidurals,” says Dr Michael Bottros, an anesthesiologist with Keck Medicine of the University of Southern California. “Someone might have a great experience with their epidural one time, and then it doesn’t work at all another time. But that doesn’t mean anyone did anything wrong.”

He says there are many variables to consider.

The epidural catheter might not be in the right spot: If the tip of the catheter isn’t in the correct location, the medication may not reach the intended areas. If this happens, it might not provide adequate pain relief, or there may be patchy coverage, experts say.

Correct placement can be difficult “because everyone’s anatomy is so different”, says Steele. For one thing, the distance from a person’s outer layer of skin to their epidural space can vary. Plus, epidural spaces themselves are uniquely shaped and sized. Other factors include body type, past surgeries, injuries, tissue structure, whether or not a person has scoliosis and nerve placement.

Even if the catheter is placed well, it can migrate during labor. “The patient may be getting in different positions, going on all fours and standing, squatting, then lying back down,” explains Steele. “Sweat can get in between the tape and the skin, and the epidural catheter may move around.”

Luckily, fixing placement issues is generally quick and easy. Gentle adjustments to the tube or helping a patient change position may help, says Steele. Otherwise, the provider may take the catheter out and start again.

The dosage may need adjustment: Providers want to give patients a high enough dose of epidural medication so they have relief, but not so much that they’re at a higher risk for side effects or complications such as low blood pressure, being too numb to feel when to push, nausea, vomiting and itchiness, Patton says.

But patients should speak up if they’re in need of more medication. Patients “may require more medication as labor progresses and contractions become stronger and more frequent,” says Patton.

In addition, people metabolize medicine differently. “Some people metabolize numbing medicine very quickly, and need higher than normal doses,” Bottros says.

Some notable studies have found this is true for people with red hair. The gene mutation that causes red hair can also cause variations in pain perception and medication metabolism. “Because of this, higher doses of local anaesthetics may be required for redheads,” says Steele.

The patient responds poorly to a particular formula: Each hospital has its own unique epidural formulation, created in response to its anesthesiology team’s experience, published evidence or data, and patient feedback. “Anesthesiology teams often develop a standard mix based on what’s worked best for their patients and clinicians,” says Habib.

Patients may respond differently to the various epidural components. Opioids improve pain relief and allow for lower doses of numbing medications, but some people experience side effects, such as itchiness, says Habib. Plus, some patients may be allergic to a particular anaesthetic or preservative, points out Bottros.

Generally, patients don’t know if they’re going to have a reaction to any of the ingredients until after the fact. For my second delivery, I didn’t know the fentanyl in my epidural would make me so itchy. (I didn’t even know this hospital’s formulation contained fentanyl until I asked.)

If patients know they shouldn’t have a particular ingredient, providers can have a custom formulation prepared – but this requires time.

The timing is not optimal: Experts say some patients mistakenly believe that epidurals will “run out” – as a result, laboring parents try to put it off until they feel they really need it. Some then find themselves in pain for hours, and risk delivering their baby before the epidural has time to kick in. (This is what happened during my first delivery.)

But this is a myth, says Steele. The epidural slowly administers medication through the catheter, she explains, and when the medication bag is empty, the provider can simply attach a new one. The whole point of epidurals is to provide a steady stream of relief for an extended time, says Steele, and it can remain in place for multiple days. “Fortunately, labor epidurals are rarely needed for that long,” she adds.

Where did this belief in the limited epidural originate? In the early 2000s, it was recommended to wait until the cervix is 4cm to 5cm dilated, partly due to concerns that early epidurals might slow labor or increase the risk for a C-section, says Habib: “However, well-conducted studies have shown that this isn’t true.”

So, there isn’t any particular stage of labor you need to wait for. “If you’re uncomfortable and feel ready, you can safely request an epidural,” Habib says.

What will give you the best chance of an effective epidural?

Preparation and communication can help. Pregnant people should carefully prepare a birth plan with their doctor and discuss potential options ahead of birth, including whether or not they want an epidural, Patton says; some providers offer options for communicating with the obstetrician anesthesia team ahead of labor. If this is an option, they can ask questions about pain relief when they’re less anxious, says Steele.

When it comes time for delivery, Steele recommends laboring patients call the anesthesia professional as soon as they get to the hospital.

“The earlier you can talk to anesthesia, the better,” says Steele, “Some people worry that if they call anesthesia, that means they have to get an epidural right away. And that’s not the case at all.”

Calling the team earlier means they have more time to learn about any requests and get consent for the epidural, making for a much smoother process.

“Every epidural is different, and they, like babies, don’t always follow the book,” says Steele.