Iron Infusion in the Third Trimester: Is It Too Late?

Iron infusion in the third trimester is one of the most common questions we hear from patients who are 32, 34, or 36 weeks along and just found out their iron is low. The worry underneath it is simple: is it too late to fix this before the baby comes? The short answer is no. The third trimester is the window where an infusion does the most good, and here is what the evidence says about timing, how fast it works, and what to expect at the visit.

Why iron runs out in the third trimester

Your blood volume grows by close to half during pregnancy, and most of that growth happens in the second and third trimesters. Your body also sends a large share of its iron to the baby in the final weeks, building the stores the baby will rely on for the first months of life. Those two demands peak at the same time. A prenatal vitamin and a reasonable diet often cannot keep up.

Ferritin, the measure of stored iron, usually falls first. Hemoglobin, the measure that shows up as anemia, follows weeks later. By the third trimester, a hemoglobin under 11 g/dL or a ferritin under 30 ng/mL means your iron needs attention, according to the international consensus definitions for iron deficiency in pregnancy. Our guide to iron deficiency in pregnancy walks through the symptoms, which are easy to mistake for ordinary late pregnancy fatigue.

Oral iron is where most people start, and for many it works. The problem late in pregnancy is time. Tablets raise hemoglobin slowly, they are absorbed poorly when the body is inflamed or the stomach is unsettled, and the weeks left before delivery may not be enough. That is exactly the situation IV iron was designed for.

Is it too late for an iron infusion at 34, 36, or 38 weeks?

No. The strongest recent evidence comes from a randomized trial published in Nature Medicine in 2025. Researchers gave 590 anemic pregnant women either a single IV dose of ferric carboxymaltose, at a median of 30 weeks, or standard oral iron. By 36 weeks, anemia was present in 47 percent of the infusion group versus 63 percent of the oral iron group. At delivery the gap was wider: 21 percent versus 35 percent. One month after birth, 36 percent versus 58 percent. No serious infusion reactions occurred.

Timing matters because of how iron works. The infusion delivers the full dose into your bloodstream in under an hour, and your bone marrow starts using it within days. Hemoglobin typically begins rising within one to two weeks, with most of the gain in place by three to four weeks. So an infusion at 34 weeks has time to change your blood count before a 39 or 40 week delivery. An infusion at 37 or 38 weeks will not fully correct hemoglobin before birth, but it still fills your iron stores, which is what your body draws on to recover from delivery blood loss and to make milk.

Clinical guidance supports this. The American College of Obstetricians and Gynecologists, in its Practice Bulletin on anemia in pregnancy, recognizes IV iron for patients who cannot tolerate or absorb oral iron and for those with severe anemia. The international consensus report goes further, recommending IV iron as the first option when anemia is severe or when there is little time before delivery. Our broader guide to iron infusion during pregnancy covers the evidence across all three trimesters.

A simple way to think about it by week:

• 28 to 34 weeks: the ideal window. Hemoglobin and iron stores can both recover before delivery.
• 34 to 37 weeks: still high value. Expect a meaningful hemoglobin rise before birth and full stores for recovery.
• 37 weeks and beyond: still worth doing. The benefit shifts toward delivery, postpartum recovery, and breastfeeding energy.

If you are anemic in the third trimester and oral iron is not moving the numbers, book a same-day iron infusion visit at Materna in the West Village. We review your labs, treat the same day when they are current, and send results to your OB/GYN.

What an iron infusion feels like, and the side effects to expect

The infusion itself takes about 30 to 60 minutes depending on the dose, followed by a short observation period. You sit in a treatment chair, a registered nurse places a small IV line, and you can read, work, or rest. Most people describe it as uneventful.

Side effects are usually mild and short. During or just after the infusion, some people notice a headache, mild nausea, flushing, a metallic taste, or soreness at the IV site. A smaller group feels achy joints or muscles, or a low fever, one to four days later. These settle on their own within a couple of days, and the NHS patient guidance on IV iron describes the same pattern.

Serious reactions are rare. Severe allergic reactions with modern iron preparations occur in roughly 1 in 1,000 to 1 in 10,000 infusions, which is why the infusion is given in a monitored setting with a clinician on site. Two other things to know: if iron leaks outside the vein it can stain the skin, so the nurse checks the line carefully, and some formulations can lower phosphate levels for a few weeks, which your clinician watches for. In the 2025 trial, 3 percent of patients had a reaction during or within 30 minutes of the infusion, and none were severe.

How fast it works, and what changes before delivery

Energy often improves first. Many patients tell us the heavy legged, out of breath feeling eases within one to two weeks. Hemoglobin follows: expect a rise over two to four weeks, with a recheck of hemoglobin and ferritin around four weeks after the infusion or before delivery, whichever comes first. Some people need a second infusion, depending on how low the starting numbers were and the dose given.

The reason this matters at the end of pregnancy is blood loss. Every birth involves some, and a cesarean involves more. Walking into delivery with full iron stores means a smaller drop afterward, a faster recovery, and more energy for the first weeks with a newborn. Postpartum anemia is also tied to low mood and exhaustion in the months after birth, which is part of why we keep following iron into the postpartum period beyond six weeks.

When to ask for an infusion rather than wait

Call your provider, or come see us, if any of the following describe you in the third trimester:

• Your hemoglobin is under 11 g/dL, or your ferritin is under 30 ng/mL.
• You have taken oral iron consistently for four to six weeks and your numbers have not improved.
• Oral iron gives you nausea, constipation, or stomach pain that you cannot tolerate, especially if you are managing hyperemesis gravidarum or ongoing vomiting.
• You are short of breath on a flight of stairs, your heart races at rest, you feel dizzy standing up, you have restless legs at night, or you crave ice.
• You are planning a cesarean or have a history of heavy bleeding at delivery.

Severe anemia, a hemoglobin under 8 g/dL, is a reason to be seen promptly rather than to try another month of tablets. Our post on anemia in pregnancy explains why it is missed so often.

What this means at Materna

When you come to us in the third trimester with low iron, we do not send you home with a stronger tablet and a follow up in a month. We look at the full picture: hemoglobin, ferritin, transferrin saturation, your symptoms, and how many weeks you have left. If IV iron is the right call, we prescribe it and give it ourselves. No referral is needed, and we work alongside your OB/GYN rather than in place of them, sending your results and plan to their office.

Timing is simple. If you have iron labs from the last 30 days, bring them and you can usually be treated the same day you are seen. If you do not, we draw them here and give the infusion the next day, once your results are back. A registered nurse administers the infusion in our West Village office, with a Materna clinician overseeing your visit from start to finish. Learn more about iron infusions in NYC at Materna or what to expect at your visit.

Iron infusion in the third trimester FAQs

Is 36 weeks too late for an iron infusion?

No. At 36 weeks an infusion still raises hemoglobin before most deliveries and fully restocks the iron stores your body uses to recover from birth. In the 2025 Nature Medicine trial, infusions given from about 30 weeks on cut anemia at delivery from 35 percent to 21 percent.

How long does an iron infusion take to work in pregnancy?

Energy usually improves within one to two weeks. Hemoglobin rises over two to four weeks, which is when we recheck your labs. The infusion visit itself takes about 30 to 60 minutes plus a short observation period.

Is IV iron safe for the baby in the third trimester?

Yes. IV iron is recommended in the second and third trimesters, and the largest recent trial found no serious infusion reactions and no safety signal for babies. Treating anemia before delivery is associated with better outcomes for both of you.

How many infusions will I need?

Many patients need only one, depending on the formulation and how low your numbers are. Some need a second dose a week or more later. Your clinician decides based on your weight, your labs, and how many weeks remain.

Can I get an iron infusion the week before a scheduled cesarean?

Yes, and it is often a good idea. Hemoglobin will not fully rise in one week, but your iron stores will be full going into surgery, which supports recovery from the blood loss a cesarean involves. We coordinate the timing with your OB/GYN.

The bottom line

Low iron in the third trimester is common, and it is fixable in the time you have left. An infusion works faster than tablets, is well tolerated, and pays off at delivery and in the weeks after. If your numbers are low and the calendar is short, book an iron infusion visit at Materna. Same-day appointments available.