Glycated Hemoglobin, or A1C, is a common blood test that provides a picture of the average blood sugar level over the preceding two to three months. This measurement reflects the percentage of hemoglobin in red blood cells that has sugar attached to it, a process called glycation. A higher A1C result typically indicates poorer long-term glucose control in non-pregnant individuals. However, pregnancy introduces profound and rapid physiological shifts that complicate the interpretation of this standard test. These unique biological factors necessitate different monitoring strategies and make the A1C a less reliable indicator of current glucose control as the pregnancy progresses.
Physiological Reasons Why A1C Changes
Pregnancy significantly alters the body’s internal environment, directly impacting how the A1C test reflects true average blood sugar levels. A primary mechanism affecting the reading is the accelerated turnover of red blood cells (RBCs). Since RBCs normally live for about 120 days, their shorter lifespan during pregnancy means less time for glucose to bind to hemoglobin. This increased destruction results in a lower, or falsely reassuring, A1C value compared to the actual average glucose level.
Another major factor is the significant increase in plasma volume, a phenomenon known as hemodilution. Blood volume can increase by up to 50% to support the growing fetus and placenta. This rapid expansion of fluid dilutes all blood components, including the hemoglobin itself. The dilution effect contributes further to a reduced A1C reading, making the test less sensitive to mild or moderate increases in blood sugar.
These combined physiological changes mean that a pregnant person’s A1C level naturally falls compared to their non-pregnant state, even if their underlying glucose control remains the same. Studies have shown that in mid-to-late gestation, the A1C can significantly underestimate the degree of glucose intolerance present. The hormonal environment of pregnancy also causes increased insulin resistance, particularly in the second and third trimesters, which leads to higher post-meal blood sugar spikes.
Interpreting A1C Readings During Pregnancy
Due to maternal physiological limitations, the role of the A1C test shifts during pregnancy. Its most valuable application is for pre-conception counseling and early screening. An early measurement provides a baseline assessment of long-term control before mid-pregnancy changes obscure the results, and is useful for screening for previously undiagnosed pre-existing diabetes.
However, the A1C becomes a secondary measure of glucose control once a person is past the first trimester. As the red blood cell lifespan shortens and hemodilution peaks, the test loses its reliability as a marker for ongoing management. Healthcare providers instead rely on alternative, more immediate monitoring methods to track daily fluctuations, such as frequent fingerstick testing (SMBG).
Continuous glucose monitoring (CGM) provides real-time data on glucose trends, allowing for immediate adjustments to diet, exercise, or insulin dosage. These daily checks give a more accurate picture of the immediate impact of meals and activity, which is far more relevant for managing moment-to-moment glucose levels than the three-month average provided by A1C. The focus shifts from a long-term average to maintaining tight, specific daily glucose targets to ensure a healthy pregnancy.
Risks Associated with Elevated Glucose Levels
Tight glucose control is necessary due to the serious risks associated with maternal hyperglycemia. Elevated blood sugar levels act as a teratogen during the first 10 weeks of gestation, significantly increasing the risk of congenital malformations, particularly those affecting the heart and the central nervous system, such as spina bifida. Even later in pregnancy, high glucose poses substantial risks to the developing fetus.
One common fetal outcome is macrosomia, where the baby grows too large for its gestational age. Excess maternal glucose crosses the placenta, causing the fetus to produce its own insulin and store the extra energy as fat, leading to excessive growth. A large baby increases the risk of complications during vaginal delivery, including shoulder dystocia, where the baby’s shoulder gets stuck after the head has delivered.
For the mother, poorly managed blood sugar increases the chance of developing preeclampsia, a serious condition characterized by high blood pressure and organ damage. Hyperglycemia also raises the likelihood of needing a cesarean section (C-section) delivery due to the baby’s size or other complications. After birth, the baby is at risk for neonatal hypoglycemia (dangerously low blood sugar) because its pancreas is still producing high levels of insulin in response to the high maternal glucose environment.
Management and Target A1C Levels
The management of glucose in pregnancy begins with achieving optimal A1C levels before conception, a time when the test is most reliable and the organs are forming. Healthcare providers recommend that women with pre-existing diabetes aim for an A1C of below 6.5% prior to pregnancy, if it can be safely reached without causing frequent low blood sugar episodes. Achieving an A1C closer to 6.0% before getting pregnant may be ideal, as it is associated with the lowest rates of adverse outcomes, including congenital anomalies.
Throughout the pregnancy, the focus shifts to maintaining strict, non-A1C-based targets for daily glucose readings, such as fasting and post-meal goals. Management typically involves intensive lifestyle modifications, including diet and regular physical activity. Many women with pre-existing diabetes will also require insulin therapy, as the hormonal changes of pregnancy often increase insulin resistance dramatically, necessitating higher doses, sometimes two to three times their pre-pregnancy dose.
The goal of treatment is to minimize the time spent in both high and low blood sugar ranges, which is why frequent monitoring is paramount. While the A1C may be used as a secondary check, the day-to-day management centers on keeping blood glucose levels within a very tight, individualized range to protect both the mother and the fetus.