Is It True That if Your First Baby Is.late Then.so.will.your Second

Are showtime babies more than likely to exist late?

Yes, and also more likely to be early. But just a picayune.

If you are significant with your kickoff child, you lot might have heard that first babies are more probable to exist late. Also, yous might have heard that they are more likely to be early. Every bit it turns out, both are true.

• If "early on" means preterm — before 37 weeks of pregnancy — first babies are more likely to exist early. Based on live births recorded in the National Survey of Family unit Growth, nigh 12% of kickoff babies are born preterm, compared to 10% of other babies.
• And if "late" means after twoscore weeks, first babies are more than likely to be late: virtually 15%, compared to ten% of other babies.

The following figure shows the distribution of pregnancy length for live births (excluding multiple births and deliveries past C-department):

Distribution of pregnancy lengths for full-term single births. The shaded areas testify 90% confidence intervals.

First babies are less likely to be "on fourth dimension" at 39 weeks, and more likely to be a petty late, between 41 and 43 weeks.

Among full-term pregnancies, first babies are born nigh one.3 days after on average. Merely the average doesn't tell the whole story.

How much longer?

Suppose you are at the beginning of Week 37. The average time until delivery at this signal is 2.eight weeks.

Two weeks after, at the get-go of Week 39, the average remaining time is 1.2 weeks. Every bit you expect, with each week that goes by, the average remaining time goes down.

But then it stops.

The post-obit figure shows the cruelest statistic in obstetrics: the average remaining time computed at the beginning of each week of pregnancy:

Average remaining time at the beginning of each week of pregnancy, for alive births, excluding multiple births and deliveries by C-section.

Between Weeks 39 and 43, the remaining time until delivery barely changes. Time goes by, but the finish line keeps moving into the future.

At Calendar week 39, if you enquire a doctor when the baby will go far, they say something similar "Any solar day now." If y'all ask over again at Week 40, they give the aforementioned answer. And once more at Week 41. That might be frustrating to hear, simply they are right; for most v weeks, y'all are always one week away.

The situation is a little worse for first babies. The following effigy shows average remaining time for first babies and others:

Boilerplate remaining time at the beginning of each calendar week of pregnancy for first babies and others.

At the beginning of Calendar week 39, the boilerplate remaining fourth dimension is 1.3 weeks for kickoff babies and 1.one weeks for others. That divergence is about 36 hours.

The gap persists for a calendar week or so, only after Week 41, first babies and others are indistinguishable.

Possibly this week?

As you plan for the final weeks of pregnancy, the boilerplate time until delivery is non very helpful. You might adopt to know, at the beginning of each week, the probability of delivering in the next seven days.

The post-obit figure answers that question for first babies and others:

Probability of delivering in the side by side week, computed at the beginning of each week.

At the start of Week 37, you can pack a bag if you desire to, but there is only a 6% chance you will demand it, first baby or non.

At the beginning of Week 38, the adventure of delivering in the next week is well-nigh 11%, not much college.

But at the beginning of Calendar week 39, it is substantially higher: 54% for first babies and 61% for others.

This gap persists for a calendar week or and so; so afterward Calendar week 41, the 2 curves are effectively the aforementioned.

Are these differences real?

The results in this article might reverberate real biological and medical differences between first babies and others. In that case, they are probable to be predictive: if you lot are expecting your first baby, you volition take to wait a little longer, on boilerplate, than for subsequent births.

Only these results might be due to measurement error.

• By convention, the duration of pregnancy is measured from the first day of the mother's last menstrual period. The reported lengths might not be precise and might be less precise for starting time-time mothers.
• As well, NSFG information is based on interviews, not medical records, so it relies on the memories of respondents. Reported lengths might be less accurate for commencement babies.

Only even if measurement errors are dissimilar for first babies, it's not articulate why they would be biased toward longer durations.

The apparent differences betwixt start babies and others might also be caused past a misreckoning factor related to pregnancy length.

• If a woman's start babe is delivered by C-section, subsequent deliveries are more probable to be scheduled and less likely to exist tardily. I excluded deliveries by C-department for this reason.
• If first babies are less likely to exist induced, more of them would exist allowed to be late. I don't know a reason they would be, but the dataset doesn't have information on induced labor, so I tin can't ostend or rule out this possibility.

The results I've presented are statistically significant, which means that if in that location were no deviation between first babies and others, we would be unlikely to see these gaps. The results are besides consistent over the course of the survey, from 2002 to 2017. So it is unlikely that the apparent differences are due to random sampling.

More reading

This article is based on a case study in my book, Call up Stats: Exploratory Data Assay in Python, which you tin can download at no cost from Green Tea Press. It is also available in paper and electronic formats from O'Reilly Media (Amazon affiliate link).

I published a similar analysis (based on older data) in my blog, Probably Overthinking It, where yous can read more articles on data scientific discipline and Bayesian statistics.

If you lot enjoyed this article, you might too like "The Inspection Paradox is Everywhere", which is virtually a surprisingly ubiquitous statistical illusion.

Methodology

I used data from the National Survey of Family Growth (NSFG), which "gathers data on family unit life, marriage and divorce, pregnancy, infertility, use of contraception, and men'south and women'southward wellness."

The dataset includes records of 43 292 live births, of which I excluded 737 multiple births and 11 003 deliveries by C-department. I as well excluded 3 cases where the duration of pregnancy was reported to be 50 weeks or more. This analysis is based on the remaining 31 906 cases.

The NSFG is representative of United States residents, but it uses stratified sampling, so some groups are oversampled. I used weighted resampling to correct for oversampling and to generate the confidence intervals shown in the figures.

The details of data cleaning, validation, and resampling are in this Jupyter notebook. The details of the analysis are in this notebook.

About the author

Allen Downey is a Professor of Information science at Olin College in Massachusetts. He and his wife take two daughters: the first was built-in a week early; the second was two weeks late, afterward a piffling encouragement.

edwardsnablowand1967.blogspot.com

Source: https://towardsdatascience.com/are-first-babies-more-likely-to-be-late-1b099b5796b6