As mothers approach the final weeks of ᴘʀᴇɢɴᴀɴᴄʏ, they typically start asking questions about how much the baby will weigh and what that means for ᴅᴇʟɪᴠᴇʀʏ. Dᴏᴄtᴏʀs think about this too, they try to accurately determine the weight of the ᴜɴʙᴏʀɴ ʙᴀʙʏ so can make the best ᴘᴏssɪʙʟᴇ ʙɪʀtʜ plan. But predicting ʙɪʀtʜ weight is actually very difficult to do.In a recent study, one-third of women reported their OB ᴘʀᴏᴠɪᴅᴇʀ told them their baby might be getting “quite large” near the end of ᴘʀᴇɢɴᴀɴᴄʏ. In the end, however, only one in five of those women had a baby that weighed more than 8 pounds, 13 ounces or 4000 grams, a common threshold for labeling a baby “large.”

It would be nice to know the exact ʙɪʀtʜ weight of a baby before it’s ʙᴏʀɴ – it makes ᴘʀᴇᴅɪᴄtɪɴɢ sᴏᴍᴇ ʀᴀʀᴇ, but sᴇʀɪᴏᴜs ᴄᴏᴍᴘʟɪᴄᴀtɪᴏɴs like ʙɪʀtʜ trauma easier. But that’s just not possible, and our estimates of your baby’s size can potentially cause unnecessary stʀᴇss on mom and lead Dᴏᴄtᴏʀs to intervene when nothing is really necessary. Here are some things to think about as you approach the end of ᴘʀᴇɢɴᴀɴᴄʏ and are wondering what your baby might weigh and how this could impact your labor and ᴅᴇʟɪᴠᴇʀʏ experience:

What is considered a ‘big baby’?

Picking an absolute cut-off is tricky. Typically, we consider estimated weights of babies that weigh more than 4500 grams (10 lbs.) as larger than normal (or “macrosomic”). But what we really want to know is whether your baby is too big for your ᴘᴇʟᴠɪs. Weight is just one factor Dᴏᴄtᴏʀs consider when we’re estimating the chances of a ᴘᴀtɪᴇɴt sᴜᴄᴄᴇssꜰᴜʟʟʏ ʜᴀᴠɪɴɢ ᴀ ᴠᴀɢɪɴᴀʟ ᴅᴇʟɪᴠᴇʀʏ.

There are three parts to the equation that determine this: “The power, the passenger, and the passage.” The “power,” or ꜰᴏʀᴄᴇ ᴏꜰ ᴜtᴇʀɪɴᴇ ᴄᴏɴtʀᴀᴄtɪᴏɴs, is something we can only assess when ʟᴀʙᴏʀ ʙᴇɢɪɴs.The “passenger” refers to the baby. His or her weight isn’t the only factor – the exact position of the baby within the ʙɪʀtʜ canal also plays an important part in making it out successfully. The direction the head is facing can make all the difference in how easily the baby descends in the ʙɪʀtʜ canal. “Passage” refers to the ᴀɴᴀtᴏᴍʏ of your ᴘᴇʟᴠɪs. When Dᴏᴄtᴏʀs do ᴠᴀɢɪɴᴀʟ exams toward the end of the ᴘʀᴇɢɴᴀɴᴄʏ, that’s what we’re trying to evaluate: How narrow is the ᴘᴇʟᴠɪs? Like the estimation of ꜰᴇtᴀʟ size, this isn’t an exact science, but it can help us determine a ʙɪʀtʜ plan for the day of ᴅᴇʟɪᴠᴇʀʏ.

Uʟtʀᴀsᴏᴜɴᴅ is not very reliable for estimating ꜰᴇtᴀʟ weight near term.

For a 9-pound baby, an Uʟtʀᴀsᴏᴜɴᴅ’s predictive accuracy is typically 15 to 20 percent off. Which means we may over- or underestimate by more than a pound. Why is there such a wide range? Uʟtʀᴀsᴏᴜɴᴅ uses volumes to calculate ꜰᴇtᴀʟ weight. It takes measurements of the head, waist circumference and some ʙᴏɴᴇs and comes up with an estimate. But it can’t measure the density of the ꜰᴇtᴀʟ tissues directly. There is a formula for calculating ꜰᴇtᴀʟ weight based on standard measurements – but not all babies follow the rules for getting the weight accurate! There is no way to know exactly how much a baby is going to weigh until after the baby is ʙᴏʀɴ.

Who is really at ʀɪsᴋ for having a big baby?

Certain health and ʜɪstᴏʀʏ factors put mothers at ɪɴᴄʀᴇᴀsᴇᴅ  ʀɪsᴋ for having a large baby:

Dɪᴀʙᴇtᴇs, including ɢᴇstᴀtɪᴏɴᴀʟDɪᴀʙᴇtᴇs, is a concern, especially if the mom’s sugar levels have ɴᴏt ʙᴇᴇɴ ᴄᴏɴtʀᴏʟʟᴇᴅ well during ᴘʀᴇɢɴᴀɴᴄʏ. High glucose levels in mom can ᴄʀᴏss tʜᴇ ᴘʟᴀᴄᴇɴtᴀ and lead to high levels in the ꜰᴇtᴜs. As a response to these high sugar levels, the ꜰᴇtᴜs produces insulin, which stimulates its own growth. Mᴀtᴇʀɴᴀʟ ᴏʙᴇsɪtʏ is another big ʀɪsᴋ factor. The rate of macrosomia has ɪɴᴄʀᴇᴀsᴇᴅ  over time with rising ᴏʙᴇsɪtʏ rates. ʜɪstᴏʀʏ of a previous big baby. The trend is for successive babies to get bigger, not smaller, so this is something we would take into consideration.

What are the concerns about having a large baby?

Immediate ʀɪsᴋs of a large ʙɪʀtʜweight for the mother are ᴅᴇʟɪᴠᴇʀʏ ᴄᴏᴍᴘʟɪᴄᴀtɪᴏɴs sᴜᴄʜ ᴀs ᴠᴀɢɪɴᴀʟ ᴀɴᴅ ʀᴇᴄtᴀʟ ʟᴀᴄᴇʀᴀtɪᴏɴ ᴏʀ ᴘᴏstᴘᴀʀtᴜᴍ ʜᴇᴍᴏʀʀʜᴀɢᴇ Long-term ʀɪsᴋs include high ʀɪsᴋ ᴏꜰ ᴘᴇʟᴠɪᴄ ꜰʟᴏᴏʀ ᴅɪsᴏʀᴅᴇʀs ᴏʀ ᴘʀᴏʟᴀᴘsᴇ. For the baby, there’s also a higher ʀɪsᴋ ᴏꜰ sʜᴏᴜʟᴅᴇʀ ᴅʏstᴏᴄɪᴀ, or when the baby’s head delivers but the shoulders don’t, and oxygen takes too long to deliver. Other potential health ᴄᴏᴍᴘʟɪᴄᴀtɪᴏɴs include low ʙʟᴏᴏᴅ sᴜɢᴀʀ ᴏʀ ᴀɴ ᴇʟᴇᴠᴀtᴇᴅ ʙʟᴏᴏᴅ ᴄᴏᴜɴt, which could lead to the baby being admitted to a neonatal all-intensive nursery.