Weight gain

Previously, I mentioned that as a new parent you are obsessed with the eating, sleeping and bathroom habits of your baby. The main reason for this obsession is to make sure that your baby is gaining weight. So the all important piece of information is `how much does your baby weigh?'

Of course, everybody reading this blog will remember (if you don't remember, this information is presented right under `About Baby Niemi' on this blog), Avalon weighed 7 lbs 8 ounces when she was born. What I did not realize and many people I have told are surprised to find out is that babies lose weight before they start to gain it back. Indeed, Avalon lost weight while in the hospital and our last measurement there was 6 lbs 13 ounces. The pediatrician also told us that if we measured her weight on the day we left the hospital, it would have decreased farther. He also suggested that if she was back up to 6 lbs 13 ounces by her first doctor's appointment, that would be good progress.

1 August 2008 Appointment - Our first appointment, as far as I understand it, was to obtain a baseline measurement of Avalon's weight. Future measurements would be compared to this baseline. From books, we know that the expected weight gain at this time is one half to one ounce per day. At this appointment, Avalon weighed in at exactly 7 lbs. Despite her only having one dirty diaper since we left the hospital (3 days ago), the consensus was that all systems were go.

4 August 2008 Appointment - We had spent the first few days with Avalon worrying about the lack of dirty diapers. At this appointment, we had spent all of Sunday changing diapers, so now we were worried if she was having too many. The clinician did not seem concerned since the color suggested that the milk was getting digested. Avalon managed to have another dirty diaper in the 15 minutes since we had changed her previously, so the clinician could see the color herself. Unfortunately, during the weigh-in, Avalon registered only 6 lbs 15.5 ounces. Although the clinician managed to hide it pretty well, I think this worried her since Avalon had lost half an ounce when she should have gained 1.5 to 3 ounces. She gave us some breastfeeding suggestions and then scheduled another appointment in two days to see how everything was going.

6 August 2008 Appointment - We, or at least I, was a bit worried about this appointment since we knew she needed to have gained weight or the docs were going to suggest something. I was heartened by the more regular dirty diapers rather than the plethora we had the previous weekend. And in fact, Avalon weighed 7 lbs 2.5 ounces at this appointment, a mere 3 ounce gain in 2 days. Our next appointment was scheduled 5 days out (2 weeks from birth), just to make sure everything was still on track.

At this point, I started wondering about the whole weighing process. I was fairly convinced that 7 lbs on the 4th and almost the same measurement on the 6th had a lot more to do with the dirty diapers than it had to do with her actual weight gain. So I started weighing her at home on our bathroom scale. Our scale seems to be pretty consistent, i.e. weighing yourself and reweighing yourself results in the same measurement. The morning before our appointment, Avalon weighed 7.2 lbs by this method. That evening, she weighed 8.1 lbs. At this point, I was a bit suspicious since her weight fluctuated almost a full pound. The morning of our next appointment, she weighed 7.5 lbs.

11 August 2008 Appointment - Judgement day! I was feeling more optimistic now since the bathroom scale said she was 7.5 lbs. Although this was our longest appointment, most of the time spent waiting for the doctor, the weight-in was done almost immediately. She weighed 7 lbs 8 ounces, exactly matching the bathroom scale measurement. Remind me again why we paid a $20 co-pay per appointment to get her weighed when our bathroom scale works just as well? The doc was sufficiently happy with her gaining 5.5 ounces in 5 days, that he gave the thumbs up. The other significance to this weight is that she is back at her birth weight and the doctors no longer suggest waking her up to make sure she gets enough calories. Of course, we have tried to let her sleep, but she still wakes up every 3-4 hours...apparently we trained her well.



All of this measurement and variability got the statistician in me thinking. In particular, I thought it was amusing that the pediatricians office always looks only at the weight gain since the previous appointment and does not use all the measurements. Since we expect her weight gain to be linear, i.e. 0.5-1 ounce per day, it is natural to use regression analysis on her four office visit measurements. The results are shown in the Figure. The important points to notice are that relative to the straight line which we might expect, the first measurement is abnormally high and the second measurement is abnormally low. So there probably didn't need to be cause for any concern. Although when all you see are the first two data points, you cannot be sure. In the end, Avalon has been gaining on average 0.86 (=0.05*16) ounces per day, so we should be happy that she is well within the expected range.