Gestation vs. Birthweight

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Birthweight (in grams) vs. Gestation time

The CDC has better data on this, but pregnant women frequently wonder whether their late babies will be huge so I asked my survey participants for weight just for funzies. On average, babies born later are slightly bigger but not much. The difference between a baby born around 39W 0D (about 3400 g) and one born around 41 W 0D (about 3700 g) is only 300 g, or about 10.6 ounces.

There's a lot of variety too; plenty of babies born after 41 weeks weigh less than babies born around week 38 or even 37. It should be no surprise that they all just seem to grow at their own rate.

Data table

Date# BirthsAverage (g)± 3 day Average (g)Change from previous week± 3 day Average (lbs oz)
35W,0D 20 2735 2765 6 lbs, 2 oz
35W,1D 23 2652 2761 6 lbs, 1 oz
35W,2D 20 2804 2753 6 lbs, 1 oz
35W,3D 32 2841 2753 6 lbs, 1 oz
35W,4D 21 2742 2781 6 lbs, 2 oz
35W,5D 30 2721 2763 6 lbs, 1 oz
35W,6D 21 2755 2773 6 lbs, 2 oz
36W,0D 49 2858 2789 24 g (0.8 oz) 6 lbs, 2 oz
36W,1D 40 2612 2811 50 g (1.8 oz) 6 lbs, 3 oz
36W,2D 48 2829 2854 101 g (3.6 oz) 6 lbs, 5 oz
36W,3D 47 2907 2914 161 g (5.7 oz) 6 lbs, 7 oz
36W,4D 40 2917 2938 157 g (5.5 oz) 6 lbs, 8 oz
36W,5D 53 3002 2969 206 g (7.3 oz) 6 lbs, 9 oz
36W,6D 70 3121 3013 240 g (8.5 oz) 6 lbs, 10 oz
37W,0D 113 2978 3035 246 g (8.7 oz) 6 lbs, 11 oz
37W,1D 101 2954 3063 252 g (8.9 oz) 6 lbs, 12 oz
37W,2D 102 3131 3093 239 g (8.4 oz) 6 lbs, 13 oz
37W,3D 113 3081 3095 181 g (6.4 oz) 6 lbs, 13 oz
37W,4D 129 3146 3137 199 g (7 oz) 6 lbs, 15 oz
37W,5D 124 3219 3171 202 g (7.1 oz) 6 lbs, 16 oz
37W,6D 159 3119 3187 174 g (6.1 oz) 7 lbs, 0 oz
38W,0D 214 3218 3224 189 g (6.7 oz) 7 lbs, 2 oz
38W,1D 196 3220 3254 191 g (6.7 oz) 7 lbs, 3 oz
38W,2D 205 3243 3268 175 g (6.2 oz) 7 lbs, 3 oz
38W,3D 238 3330 3303 208 g (7.3 oz) 7 lbs, 5 oz
38W,4D 303 3334 3322 185 g (6.5 oz) 7 lbs, 5 oz
38W,5D 286 3318 3346 175 g (6.2 oz) 7 lbs, 6 oz
38W,6D 335 3382 3359 172 g (6.1 oz) 7 lbs, 6 oz
39W,0D 476 3348 3378 154 g (5.4 oz) 7 lbs, 7 oz
39W,1D 422 3400 3395 141 g (5 oz) 7 lbs, 8 oz
39W,2D 388 3378 3410 142 g (5 oz) 7 lbs, 8 oz
39W,3D 504 3444 3431 128 g (4.5 oz) 7 lbs, 9 oz
39W,4D 512 3451 3464 142 g (5 oz) 7 lbs, 10 oz
39W,5D 495 3443 3484 138 g (4.9 oz) 7 lbs, 11 oz
39W,6D 545 3522 3506 147 g (5.2 oz) 7 lbs, 12 oz
40W,0D 1002 3519 3524 146 g (5.1 oz) 7 lbs, 12 oz
40W,1D 638 3554 3541 146 g (5.1 oz) 7 lbs, 13 oz
40W,2D 581 3570 3562 152 g (5.4 oz) 7 lbs, 14 oz
40W,3D 536 3597 3576 145 g (5.1 oz) 7 lbs, 14 oz
40W,4D 521 3590 3603 139 g (4.9 oz) 7 lbs, 15 oz
40W,5D 542 3620 3620 136 g (4.8 oz) 7 lbs, 16 oz
40W,6D 564 3635 3639 133 g (4.7 oz) 8 lbs, 0 oz
41W,0D 601 3659 3655 131 g (4.6 oz) 8 lbs, 1 oz
41W,1D 498 3666 3671 130 g (4.6 oz) 8 lbs, 1 oz
41W,2D 394 3725 3690 128 g (4.5 oz) 8 lbs, 2 oz
41W,3D 396 3727 3704 128 g (4.5 oz) 8 lbs, 3 oz
41W,4D 295 3719 3724 121 g (4.3 oz) 8 lbs, 3 oz
41W,5D 238 3790 3742 122 g (4.3 oz) 8 lbs, 4 oz
41W,6D 200 3725 3751 112 g (4 oz) 8 lbs, 4 oz
42W,0D 188 3786 3772 117 g (4.1 oz) 8 lbs, 5 oz
42W,1D 108 3777 3792 121 g (4.3 oz) 8 lbs, 6 oz
42W,2D 65 3839 3797 107 g (3.8 oz) 8 lbs, 6 oz
42W,3D 47 4041 3831 127 g (4.5 oz) 8 lbs, 7 oz
42W,4D 19 3891 3851 127 g (4.5 oz) 8 lbs, 8 oz
42W,5D 20 3921 3895 153 g (5.4 oz) 8 lbs, 9 oz
42W,6D 12 3898 3926 175 g (6.2 oz) 8 lbs, 10 oz
Total responses: 13956

Due Date Survey Data

Due date statistics: A study on the length of pregnancy
Probability of delivery resulting from spontaneous labor after 35 weeks
Probability of delivery within x days of a given date
Length of pregnancy by week
Spontaneous labor and due date determination
Length of pregnancy, comparing subsequent births for individual moms
Length of pregnancy for first time vs. second & third time moms
Length of pregnancy, type of delivery
Gestation vs. Birthweight
Probablity of Induction after a given day
Average day of spontaneous labor vs. age of mother at time of birth
Are more babies born during a full moon?
What's the most common day of the week for babies to be born?
I'm still pregnant at 40W. What's it mean?
Do winter babies arrive later?
What if I know my conception date?
Are boys or girls born later?
Subsequent pregnancy date search
How do inductions bias the due date statistics?
Download the data
Birth Stories
Survey input dates

More Resources

Pregnancy Day-by-Day
Fast pregnancy calendar
Baby age calendar
EDD Icon Generator
Jo's Birth Story
Weeks vs. Months Explained
Charts generated by flot

Recent Comments

For more support, encouragement, Q&A,
you are invited to join our pregnancy forum. We are dedicated to answering complicated questions as logically as we possibly can.

kids' mail subscriptions

Thu Dec 13 1:56 AM by rowan in Pregnancy & Parenting

I don't know that specific Kiwico line, they only had two lines when I looked at them last. We did their Tinker Crate for a couple years and enjoyed them but after a while we just didn't get to them and we still have a shelf full of unopened boxes.

housemates + newborn = ?

Thu Aug 2 11:57 AM by melsbells in Pregnancy & Parenting

I live with my in-laws, and while there will certainly be a lot of differences from your situation, maybe there's enough similarities that my experience could be useful to you. I think the biggest thing is that everyone has their own space. Our house is sort of divided in two, so we can take the kid to our end during a tantrum. I think that distance helps, not just with in-laws not wanting to hear screaming, but as a sort of physical signal for the kid to calm down. We lived in an apartment when the kid was first born and our neighbors there probably heard more screaming/crying due to proximity.

My father-in-law spends a lot of time with the kid, mostly by choice. I sometimes wish we had more of a schedule, because at the moment, naps and bedtime are the only semi-guaranteed breaks for anyone. My spouse and father-in-law both work from home free-lance, so they don't actually have a schedule for when they need to be left alone, but at least my father-in-law has more warning with jobs scheduled out in advance.

My Ectopic Pregnancy

Tue Jan 23 8:47 PM by rowan in Pregnancy & Parenting

FYI we have a general culture in the pregnancy forum here about sharing similar stories as a way to offer support, and also about giving any medical knowledge that we've come across, since pregnancy is generally not talked about as much as it should be and you find information in random places. Since you haven't been around until now you probably didn't know that, and mels' post is well in line with forum guidelines and norms. Please don't go all mod martyr on us now that you're only just back. This is me as a friend speaking here, not with my modhat on.

I'm sorry for your loss. Ectopic pregnancies are scary, and I would like to go punch that first doctor in the face for you.

PPD is awful too. :( I hope you can get through it.

gender recognition

Thu Nov 2 1:03 PM by Nech in Pregnancy & Parenting

You can continue discussing with him the difference of a boy and a girl. Have him socialize too with the same gender. There's nothing wrong if you have go with opposite sex as long as he understands what/who he is.

So do you feel hanging out with the opposite gender at a young age can confuse a person's gender identity?

leftist parenting and gender [split from gender recognition]

Sat Sep 23 11:52 AM by Unvoiced_Apollo in Pregnancy & Parenting

gender recognition.">

If a boy who has a penis asks if he's a boy and you don't just say yes, then that's straight up lunacy.

What he decides to do with that boyhood or how he dresses is up to him. But if you're born with a penis you're a boy.

Making it airy fairy when, statistically, scientifically, less than 1% of 1% of children born have true gender dysphoria, is immoral and wrong. It's also a sign of our troubled times.

This is just my anecdotal experience but...

I have experienced bullying based on simply engaging in normal children's play with girls. Insults were attacks on both me and the girls. But I NEVER questioned my gender as a male. So if a child asks me if he's a boy and was identified biologically as one, I am going to question what influences are on the child before I answer.

How important is school?

Thu Nov 22 9:05 PM by Skeezy in Pregnancy & Parenting

I think the most important factor in any school is how much the teachers care about the students. Another important element is safety and how interactions between students can substract from learning. Many parents go to jail for quite lengthy sentences for trying to send to their kids to a safe good school.

I would say its very important, because what you learn shapes and molds yours views that make you who you are.

Unplanned, polyamorous pregnancy situation

Sun Apr 29 12:18 PM by metawidget in Pregnancy & Parenting

I'm glad you have been communicating and working things out. I wish you lots of health and love as you wrote the next chapter of this story.

I might be pregnant...

Fri Jan 5 3:16 AM by Aum in Pregnancy & Parenting

Aum, my partner and I were just discussing this. He was surprised by my taking the hard line that stopping BC or poking holes in condoms or something like that is rape, making the woman a rapist. Which should result in jail time, so she should lose custody, which should go to the father and now he has the choice of keeping the baby or putting them up for adoption.

It's unfortunate that you can't prove such a thing, but hopefully if it was actually taken seriously women would be less likely to do it because it would at least be explicitly called rape.

I understand how the justice system and the family courts would look at it. They see it as the man's sperm made it to the egg so somehow he wasn't protecting himself, he made the choice, yada yada. There's no way to prove that the woman was manipulative, withdrew BC, or "poked holes in the condom" (I think that's unheard of, but anyway).

When do you outsource parenting?

Sat Oct 14 7:47 PM by rowan in Pregnancy & Parenting

yeah I do a ton of supplemental history... :(

It might be time for a baby.

Sat Sep 16 1:59 PM by Sonic# in Pregnancy & Parenting

I got curious on the question of single parent families. Definitely, I think financial stability or having some sort of support network (family, friends) is better than not having it.

I found this article ( ). The article does point out that several factors adjust child adjustment, like finances and parental stress. Merely being a single parent? Not a factor:
The findings suggest that solo motherhood, in itself, does not result in psychological problems for children.

The lit review at the beginning of the article adds some specificity here. What seems hard on kids is not single parenthood itself; it's often the divorce that results in a single parent being involved. These studies have consistently shown that children whose parents divorce are more likely to show emotional and behavioral problems than are children in intact families (Amato, 2000, 2001, 2005; Coleman & Glenn, 2009; Hetherington & Stanley-Hagan, 1999; Pryor & Rodgers, 2001). However, the children’s difficulties appear to be largely associated with aspects of the divorce, rather than single-parenthood, in itself. Then there are people who become single parents, but not by choice. That's also rough, often because these parents lack the financial stability and stability to do well. So the article is really looking at single parents by choice. They can raise children as well as two-parent households.

How the 2nd pregnancy / labor was different from the first

Thu Sep 27 8:20 PM by antfancier in Pregnancy & Parenting

I haven’t been around for a long time, but I’m back and pregnant again. Only 6 weeks, so I can’t really comment much, but this time around it has been easier so far. Not easy, but considering I’m working and have a 2.5yr old (neither of which applied to last pregnancy), I thought I’d feel much worse. I still have ALL the symptoms and they all started at 3 weeks, just like last time (and the same as a chemical pregnancy I had too) but they don’t seem to be as constant as before. I’m really glad about that! The only thing that’s worse is back ache, ligament pain, general body aches. I assume it’s because my body is already exhausted and broken from my toddler.

posting pictures of kids and privacy

Wed Apr 25 7:29 AM by juliabonl17 in Pregnancy & Parenting

Usually, we don't share photos of our children in social media. Sometimes I can send some photos to close friends. I keep my personal life in quiet. Share with family only.

I had my 2nd baby!!!!

Wed Jan 3 8:21 PM by MFS in Pregnancy & Parenting


I am still dealing with the fact that your 11 year old is an actual 11 year old.

on being semi-pregnant

Tue Oct 3 7:18 PM by geldofc in Pregnancy & Parenting

congratulations!! i've never been pregnant but i'm jealous. i'm having baby fever lately.


Wed Sep 13 6:58 PM by geldofc in Pregnancy & Parenting

my clock started ticking in my 20s and i would love a child at 30 now but i want to live how i want for a few more years, and have my career established. i don't see parenthood for another 5 years.

egg freezing seems like a good option esp if you have extra time and money.