Most women with gestational diabetes manage their blood sugar with diet and exercise alone. Insulin becomes necessary when those lifestyle changes can’t keep blood sugar below specific targets, typically a fasting level under 95 mg/dL and a one-hour post-meal level under 140 mg/dL. This usually becomes clear within one to two weeks of consistent blood sugar monitoring after diagnosis.
The Blood Sugar Targets That Drive the Decision
After a gestational diabetes diagnosis, you’ll start checking your blood sugar multiple times a day, usually fasting in the morning and one or two hours after each meal. The numbers your care team watches are straightforward: fasting glucose should stay below 95 mg/dL, and your reading one hour after eating should be below 140 mg/dL (or below 120 mg/dL if your provider uses the two-hour post-meal check instead).
If your readings consistently exceed these targets despite following a meal plan and staying active, that’s the trigger for medication. “Consistently” typically means more than three elevated readings in the same week. A single high number after a holiday dinner doesn’t mean you need insulin. Your provider is looking at patterns over days, not isolated spikes.
How Long Diet and Exercise Get Before Medication Starts
Most providers give lifestyle changes a trial period of one to two weeks after diagnosis. During that window, you’ll follow a structured eating plan that spaces carbohydrates across meals and snacks, and you’ll aim for regular moderate activity like walking after meals. Your blood sugar logs from this period tell the story. If your numbers fall into range, you continue with diet and exercise alone, and many women do successfully manage this way throughout pregnancy.
If your fasting numbers are stubbornly high, that’s often the hardest to fix with food alone, since fasting glucose reflects what your liver does overnight rather than what you ate. High fasting readings are one of the most common reasons insulin gets added. Post-meal spikes, on the other hand, sometimes respond well to adjusting portion sizes or swapping certain carbohydrates, so your provider may fine-tune your meal plan before jumping to medication.
Why Your Baby’s Growth Can Change the Timeline
Blood sugar numbers aren’t the only factor. Ultrasound measurements of your baby’s growth, particularly the abdominal circumference, can push the decision toward insulin even when your glucose readings look borderline acceptable. When the baby’s abdominal circumference measures at or above the 75th percentile for gestational age, it signals that the baby is being exposed to more sugar than the numbers on your meter suggest. Research has shown this threshold catches cases of excessive fetal insulin production that blood sugar logs alone would miss.
Some centers now use a flexible approach: if the baby’s abdominal circumference is below the 75th percentile, they allow slightly more relaxed blood sugar targets. If it’s at or above that mark, they tighten targets significantly and are quicker to start insulin. In one clinical trial, this approach led to insulin being started in about 4% of women whose blood sugar numbers alone wouldn’t have warranted it, but whose babies were growing too fast.
Insulin vs. Oral Medications
Insulin is considered the standard first-line medication for gestational diabetes because it doesn’t cross the placenta, meaning it has no direct effect on the baby. Oral medications like metformin do cross the placenta, and while studies have shown comparable outcomes for blood sugar control and pregnancy complications, the long-term effects on children exposed in utero are still being studied.
That said, oral medications are widely used and considered safe alternatives, particularly for women who struggle with injections or have difficulty managing an insulin regimen. Patient satisfaction tends to be higher with pills for obvious reasons. One important caveat: among women who start on oral medication, a significant portion (up to 46% in some studies) eventually need insulin added anyway because the pills alone can’t keep up as pregnancy progresses and insulin resistance naturally increases.
Your provider will weigh your blood sugar levels, how far along you are, your baby’s growth, and your preferences when recommending one approach over the other.
What Happens If Insulin Is Delayed Too Long
When blood sugar stays elevated and treatment doesn’t escalate quickly enough, the risks to both mother and baby increase. The baby receives excess glucose through the placenta and responds by producing more of its own insulin, which acts as a growth hormone. This leads to macrosomia, meaning a birth weight over 4,000 grams (about 8 pounds 13 ounces), which raises the likelihood of birth injuries, emergency cesarean delivery, and shoulder complications during vaginal birth.
Persistently high blood sugar can also cause the baby to produce more urine, increasing the volume of amniotic fluid, a condition called polyhydramnios. In women with gestational diabetes and excess amniotic fluid, the rate of having a larger-than-expected baby roughly triples compared to those with normal fluid levels (about 20% vs. 5% for macrosomia). These pregnancies also carry higher rates of cesarean delivery, NICU admission, and in rare cases, stillbirth. This is why providers don’t wait long before adding medication once it’s clear diet and exercise aren’t enough.
What Using Insulin Actually Looks Like
If you do need insulin, the regimen is tailored to where your numbers are running high. If only your fasting glucose is elevated, you’ll likely start with a single injection of long-acting insulin at bedtime. Long-acting formulations work gradually over 20 to 24 hours, smoothing out overnight blood sugar without causing sudden drops. If post-meal numbers are the problem, you’ll use a rapid-acting insulin before the meals that cause spikes. Some women need both types.
The doses start small and get adjusted frequently based on your blood sugar logs. You’ll check your levels at least twice daily (and often more, especially early on) and share those numbers with your care team, who may adjust your dose every few days. Insulin needs tend to increase as pregnancy progresses because placental hormones that drive insulin resistance peak in the third trimester. A dose that works at 30 weeks may need to be raised by 34 or 36 weeks.
The injections use very small needles, typically into the upper arm, thigh, or abdomen (avoiding the area right around the belly button). Most women find the injections far less painful than they expected, and the routine becomes second nature within a few days.
After Delivery
Insulin for gestational diabetes is almost always temporary. Once the placenta is delivered, the hormones driving insulin resistance drop rapidly, and most women stop insulin immediately after birth. Blood sugar is monitored in the hours following delivery to confirm it normalizes. Your baby’s blood sugar will also be checked, since newborns who were exposed to high glucose in utero can experience a temporary dip after birth when the sugar supply from the placenta suddenly stops.
Having needed insulin during pregnancy does increase your risk of developing type 2 diabetes later in life compared to women who managed with diet alone. A glucose test at 6 to 12 weeks postpartum, and periodic screening in the years that follow, helps catch any lasting changes early.