Most of the research we are interested in as midwives is inferential – meaning we draw conclusions about a group of people based on the results. However, descriptive statistics are still very helpful.
Descriptive statistics help us organize and summarize information. For example, the number of births attended by midwives is a descriptive statistic. We can break down the data by country or state/province and see differences between groups. In experiments, the descriptive statistics help us ensure the two study groups are similar.
In healthcare, statistics isn’t useful without epidemiology. Epidemiology is the study of patterns of illness and conditions. We use epidemiology to determine the causes of conditions, the effects of exposures and treatments and the patterns of spread for health issues.
In epidemiology, there are two terms to be familiar with for descriptive statistics: incidence and prevalence. Let’s review these first with a non-health outcome – the number of midwives.
Incidence is a measure of new cases of something. For example, according to the American Midwifery Certification Board, the incidence of first time candidates for the midwifery certification exam has increased from 297 in 2005 to 542 in 2013. This is the number of new midwives each year.
Prevalence is a measure of the total number of cases of something, this means the total pre-existing and all new cases. For example, according to the North American Registry of Midwives , the prevalence of certified professional midwives had increased from 624 in 2000 to 1828 in 2010. This is the total number of midwives, both new and existing.
Take a look at this article: Prevalence of Hepatitis B Virus Seromarkers in Young Adults Vaccinated at Birth; Impact on the Epidemiology of Hepatitis B Infection in Iran
In this study, the researches wanted to see if the infant immunization schedule for Hep B was successful at reducing infection rates. To do this, the researchers tested a group of young adults to see what types of Hep B antibodies they had (from vaccination, from cleared infection or from chronic infection). This means the researchers were looking at prevalence, or the total number of people who test positive for each particular type of antibody.
Birth Worker Survey
The Birth Worker Survey allows us to gather some descriptive statistics about the services offered by the readers of the Birthing Naturally website. Remember, our total was 31 responses. Here are descriptive statistics for the most commonly reported services.
Provide Midwifery Services: n= 6 (19%)
Provide Doula Services: n=24 (77%)
Provide Childbirth Education Services: n=16 (52%)
Provide Breastfeeding Education: n=15 (48%)
Provide Herb or Essential Oil Blending: n=5 (16%)
Provide Labor Photography: n=5 (16%)
Provide Placental Preservation: n=4 (13%)
In the next post, we’ll talk about some common problems with presenting data.
A big part of understanding the statistics in research is understanding what is behind the data. What the data actually is, and how you obtain the data determine the types of statistical tests you will use. If you have been reading research, you probably already have an idea about most of the terms we will talk about today. But just in case, today we will make sure everyone is on the same page.
The most basic element of data is the variable. A variable is a measurement that represents a characteristic. Some examples of variables would be age, gender, height, parity, estimated fetal weight, and fetal heart rate. Each of these characteristics will be different for different individuals. It is these differences that interest us. How are they different? Why are they different? Do the differences matter?
Not all variables can be treated the same statistically. Characteristics of the variable itself determine what types of statistics can be used. All variables will be either continuous or discrete.
A continuous variable can take any value between its minimum and maximum. Characteristics such as age, weight, and blood pressure are continuous variables. Continuous variables are measured with a number, and the differences between each number are proportional meaning the magnitude of difference between 1 and 2 is the same as the magnitude of difference between 3 and 4.
Some continuous variables are interval and others are ratio. The difference is that a ratio variable has a specific zero measurement which indicates there is none of that variable. A gravid 4 has had twice as many pregnancies as a gravid 2, and a gravid 0 has had zero pregnancies. This means gravidy is a ratio variable.
A discrete variable is any variable that is not continuous, meaning it is only able to take on specific values. Characteristics such as race, sex, and use of oxytocin in labor are discrete variables. There are several types of discrete variables that may be used in research. Discrete variables are also known as categorical variables.
Some discrete variables are dichotomous, meaning there are only two options. Usually the options are yes or no.
Some discrete variables are nominal, meaning the characteristic has multiple values but the values do not have a mathematical relationship. For example, state of residence is a categorical value. Each individual may have a different state of residence, but there is no inherent ranking of the states. Other categorical variables include race and ethnicity, marital status, and place of birth.
Other discrete variables are ordinal, meaning the characteristic has multiple values that have a mathematical order without a defined magnitude of difference. For example, the pain scale asks women to rate the pain they feel as a number between 0 and 10. In this scale an answer of 10 is more pain than an answer of 9, but the difference between 9 and 10 may not be the same as the difference between 2 and 3. Similarly, the measurement of a “9” may not be the same from woman to woman.
Birth Worker Survey
The Birth Worker Survey included all types of variables. Age is a continuous ratio variable. Gender is dichotomous. Race is nominal. The questions about beliefs about childbirth are ordinal. This matters, because you report information about the variables differently.
50% of the respondents indicated they provide childbirth education services (a dichotomous variable). Childbirth educators were asked about the hours per session taught, number sessions in a typical course, and the number of courses taught per year. These are all continuous variables. Researchers using continuous variables will report the maximum value, minimum value and the mean value.
Hours per Session: Minimum 1.5; Maximum 10; Mean 3.1
Sessions per Course: Minimum 1; Maximum 12; Mean 6.4
Courses per Year: Minimum 2; Maximum 20; Mean 6.4
Childbirth Educators were also asked about the organizations they trained or certified with. This data is discrete, so it should be reported as a frequency. Frequency is reported as both the number of responses, percentage of the total. In research, these values would also be listed with measures of spread (confidence interval), but we will talk about those in a future post. Here is a partial list of the most frequently cited organizations.
American Academy of Husband Coached Childbirth n=4 (16%)
International Childbirth Education Association n=3 (12%)
Lamaze International n=3 (12%)
Childbirth and Postpartum Professionals Association n=2 (8%)
Childbirth International n=2 (8%)
Hypnobirthing n=2 (8%)
Spinning Babies n=2 (8%)
In the next post we will learn about descriptive statistics.
When I first began to read original research papers, I would skim over the statistical part to get to the conclusions. I understood statistics enough to tell if group A and B were different, and since the rest didn’t make sense to me I skipped it. I’m a little wiser now, and have a strong base of statistical knowledge to inform my reading. Honestly, it changes the research reading completely.
Statistics is simply the study of numerical data – how to collect it, analyze it and interpret it. In research, we use statistical testing to determine if the results of a study show a true phenomenon or were the result of chance.
Statistics can be broken down into two broad categories – descriptive or inferential. Descriptive statistics allow us to organize and summarize information from data. Inferential statistics lets us use a sample to draw conclusions about a population.
The population is the group of every individual you are interested in. For example, you may want to know about all the women of childbearing age in the United States. Or, you may want to know about all the women of childbearing age in the United States who were born in Mexico and primarily speak Spanish in their home. Both are populations and would be legitimate for a study. As the researcher, you define what population the study will examine.
The sample is the group of individuals you are able to collect information about. In inferential statistics, this group of individuals allows you to make estimates about the population.
Answering a Question
We like statistics because it helps us to answer a question. But for statistics to be useful we need to create a very specific question. The question is about the relationship between two variables. The Independent variable is the characteristic of interest, often thought of as the exposure. The Dependent variable is the outcome that depends on the independent variable.
Birth Worker Survey
The Birth Worker Survey can give us an example of independent and dependent variables.
Starting with the basics, we had 31 completed surveys. This is helpful statistically because 30 is a target number for being able to make assumptions about the normality of the mean of a variable, but that gets very technical and beyond what you need to know. Just be aware that all my hounding you to respond gave us a unusable sample for analyzing.
Our n (or size of the sample) is 31. Of those 24 women (incidentally, the respondents were all female) reported they do now or did in the past work as a doula.
If you remember, the Beliefs about birth questions were ranked 1-5, with 1 being strongly agree and 5 being strongly disagree. The simplest method to compare the two groups is to take the mean of the scores (the average of the scored values). When we do this, we find the doula group has a mean score of 1.912, indicating the doula group agrees that women should have a doula. The non-doula group has a mean score of 2.286, closer to an indifferent score.
So, is the dependent variable related to the independent variable? The means were different, but we have such a small sample that using this test we don’t get a statistically significant difference. The p-value is only .365, and the confidence interval for the difference between the means goes from -1.189 to 0.4509. This data and this test have not given evidence of a difference in belief about women having a doula in labor between doulas and other birth workers.
If you got a little lost, don’t worry. This is just day one, we will take time to discuss all of this later.
Point to Remember
While statistics is helpful for identifying the difference between a true phenomenon and a random result, it is important to remember statistics are only one piece of the design of a study that help you determine if findings are valid. The math can be good, and the result can be poor simply because the wrong sample was used or the wrong data were collected. We won’t go into all the aspects of a good study this summer, but perhaps we can plan for a series on research in the future. Later this week, we will start exploring data.
In the next post we will begin to explore the variable.
I did not want to leave the topic of obesity without talking about how it affects midwifery practice. Because of the increased cost, time and risk involved in providing care to an obese woman, some obstetricians have BMI restrictions for their practice – instead requiring women with high BMIs attend a high risk, maternal fetal medicine practice.
If you work with obese women into your practice, you can expect on average 1 in 3 will need something beyond standard care. How that affects your practice will depend on many things – like your fee structure, your physician back-up, your ability to admit women to a hospital. This makes for some very difficult decisions on the part of a midwife. Continue reading
This week we have been talking about pre-pregnancy obesity. We looked at the increasing rates of obesity, then talked about the physiology of obesity and how it increases risks during pregnancy. Today we will turn our attention to what midwives can do. How do we provide the best evidence-based care for obese women?
Midwives don’t seem to be very good at dealing with obesity. Some researchers asked a group of midwives to recruit women with a BMI >30 into a study on the effects of a dietary intervention. But the researchers were actually interested in what the midwives would do. During the four months of recruitment, the midwives only talked 14% of the eligible women. Why? The midwives said they had personal and professional reasons that made them reluctant to talk about obesity with the women.
I get it. You probably get it too. It’s rude. It is embarrassing for the woman. She might be offended. She might get angry. Midwives are supposed to be encouraging, but being told you are obese might undermine the woman’s confidence in her body. But to ignore her health risks is unfair to her. Continue reading
The problem is that reducing risk is out of the midwives hands. Really, the answer seems to be that you have to somehow convince the woman to make the dietary and lifestyle changes to stay within the Institute of Medicine (IOM) weight gain guidelines.
Coming to Terms with the Guidelines
I have to tell you, when I first heard about the guideline changes I was angry and convinced the science had to be bad. How can the IOM recommend that women gain less weight than the baby and placenta and fluid weigh. This is essentially recommending that women diet during pregnancy and I was raised in the natural birth world where pregnant women don’t diet. This had to be a mistake. Continue reading
We have been discussing pre-pregnancy obesity, and today we turn our attention to the effects of obesity on pregnancy. Before we begin, I just want to make sure we recognize the difference between pre-pregnancy obesity and excess gestational weight gain. Both bring increased risk to pregnancy, but they are different risks.
Which brings up another point to remember, these are only risks which are measured by the odds of something happening. A risk is not an absolute, it is only an estimate of the potential for a problem. The odds of an event happening is measured by looking at a large number of people and seeing how many have any specific problem. But an individual doesn’t get 30% of a problem – they either have the problem or they do not. This is risk.
When I read popular literature about obesity and pregnancy, I read about a very limited set of problems. These problems begin with the labeling of obesity as a risk category for pregnancy, which allows the mother fewer options and results in unnecessarily high cesarean rate for obese women. My concern with this line of reasoning is not that it is necessarily wrong, but that it is dreadfully incomplete and gives the false impression that obesity adds no REAL risks to the health of mother or baby.
Pregnancy Induced Hypertension and Gestational Diabetes
We know the adipokines (hormones produced by adipose tissue) increase risk for hypertension, and we see this remains true in pregnancy and postpartum. Women with a BMI >30 have a diminished vascular responsiveness during pregnancy. And Instead of a normal postpartum dip in blood pressure, their blood pressure seems to be reset at a higher level. Estimates of hypertension in pregnancy increase with BMI from less than 1% to about 17% as BMI increases.
In fact, the odds of pregnancy hypertension appears to increase with increasing BMI. So while overweight women have an odds ratio of 1.99 (about twice as likely to have pregnancy induced hypertension), women with BMI >40 have an odds ratio of 4.26 or more than 4 times the odds of developing hypertension. In this study on women with BMIs over 40 (33% were having their first baby), they found 1/3 of the women were admitted to the hospital during their antenatal care – 61% of those admissions were for hypertension treatment.
This still means about 80% of women with BMIs >40 will not develop PIH. But remember, 20% is the same as 1 in 5. This means that while on average 1 out of 100 normal weight women will develop PIH, 1 in 20 obese women and 1 in 5 women with a BMI >40 will develop PIH.
Just like hypertension, the odds for gestational diabetes increases with increasing BMI. Women with a BMI over 40 had over 3 times the odds of developing gestational diabetes. This means that while we expect about 1 in 25 normal weight women will develop gestational diabetes, about 1 in 10 obese women will develop gestational diabetes.
And these numbers are probably low estimates. A different study found the prevalence of gestational diabetes was 19% in women with BMI of 40 or higher. This means 1 in 5 women with BMI at or above 40 developed gestational diabetes in this study.
We can debate appropriate cut-off levels for hypertension or diabetes. We can be frustrated that we don’t have better treatment options for women. But we cannot ignore that the physiological effects of obesity change a woman’s ability to maintain her health in pregnancy.
What about other risks?
Studies repeatedly show risks such as sepsis and thrombo-embolism have a higher odds of occurring for obese women. And this makes sense when we consider the effect of the adipose hormones. Remember they play a role in immune function and clotting factors – so we could expect if the levels of fat hormones are out of whack we would see more infection and thrombo-embolism problems.
Particularly frightening is the strong associations between obesity and preeclampsia. The odds ratios are the same as the Pregnancy Induced Hypertension ratios with women in the highest BMI category more than four times as likely to develop preeclampsia. And while a nullipara had to have BMI>40 to be in the highest risk group, a multip only needed to have a BMI>30.
Remember, some women will only have one problem while others will have multiple. The best estimates are that 1 out of 3 obese women will not have a normal pregnancy. That means 2 out of 3 will continue without problems.
And, we know obese women are more likely to give birth via cesarean, with severely obese (BMI>40) with at least 3 times the odds of giving birth via cesarean. Yes, there is a difference in elective cesarean, but the rates of emergency cesarean (or decision to cesarean made after labor has started) are actually higher than the elective cesarean group.
One of the problems for an obese woman is that it is going to take her longer to labor. It is not a difference in uterine contractility. The contractions themselves take longer to open the cervix in obese women. In fact, it even takes longer for an obese woman to even start labor. So there is one silver lining – obese women actually have a lower rate of spontaneous preterm birth.
I could not find hypotheses about why the contractions of the same strength are not effective on the cervix of an obese woman. But regardless of the reason, it takes about 1.2 hours more for an obese nullip to reach 10cm. And because of this “slow to start” labor reality, an obese woman is 2.5 times more likely to be induced – but this is also due to the higher rates of induction for preeclampsia.
But the higher rates of cesarean add another concern for obese women. The odds of epidural failure is higher, but so is the failure rate for tracheal intubation to allow for general anesthetic. Higher rates of wound infection have led to recommendations for higher or longer doses of prophylactic antibiotics and different surgical closure methods. As we said, an obese woman has much higher odds of a clot. And when she has her next baby, her odds of VBAC failure nearly double.
Risks for the Baby
Obese women are more likely to have infants with congenital deformities like neural tube defects and congenital heart disease. The odds for congenital heart disease increase with increasing BMI, so while there is a 25% increase for women with BMI 30-39, there is a 50% increase for women with BMI >40.
Obese women have at least a 50% increase and possibly over 200% increase in odds for neural tube defects. In fact, in the UK guidelines now recommend a 5mg daily folic acid supplement for pregnant women with BMI >30.
But dysfunctional placentas are not only affecting development. They seem to be affecting the ability of the baby to survive. It turns out, stillbirth is nearly twice as likely in obese compared to non-obese women – In the UK (where better public health records are kept) almost 1/3 of the women who have a stillborn infant or dies in the neonatal period is obese.
The increased risks for both mother and baby with pre-pregnancy obesity are real. But remember, these are risks – not absolutes. Tomorrow we will talk about what midwives can do to help women reduce the risks.
Before we go any further, I want to make sure we are all defining obesity the same. Obesity is defined as an abnormal or excessive fat accumulation that may impair health. In pregnancy, we use the pre-pregnancy BMI estimate to assess overweight and obesity.
One of the things to understand when you read news articles is that obesity can be measured in several ways. You can use waist circumference or measure percentage of body fat, or you can ask people to report their height and weight and calculate a BMI. Continue reading
I promised last month to come back to our discussion about obesity and midwifery, and I am keeping that promise. Our discussion is going to stay focused on pre-pregnancy obesity, and we will start today by discussing the increase in obesity. Tomorrow we will begin to talk about the increased risks for mother and baby, along with the physiology behind those risks and how to reduce risks. Friday, we will wrap up our discussion by considering how pre-pregnancy obesity affects midwifery care.
So to start, let us consider some surveillance information from the Centers for Disease Control and Prevention (their public data is published in maps so it is easy to see changes). Scroll down a bit and check out the slideshow for the History of State Obesity Prevalence. You can stop the slideshow and move to the first slide representing 1985 and notice how no state had an obesity rate greater than 15%. Slowly move the slideshow forward and watch what happens once you pass 1990. Continue reading
A Study out of the University of the Sunshine Coast is looking for parents, both men and women, over 18 years old who feel the birth experience with their child was traumatic.
From the website:
The volunteers will be invited to complete an online survey containing 121 questions which will take approximately 20 to 30 minutes. The survey will investigate previous mental health, symptoms of posttraumatic stress disorder, coping strategies, parenting self-efficacy, and relationship with spouse quality. Questions include asking about anxiety, depression, recurring memories of the traumatic experience, and any positive and negative feelings about your baby.
If you think your child’s birth was traumatic, and this sounds interesting, I encourage you to head over to the survey and start filling it out. As a research participant, you always have the right to change your mind or stop completing the survey if it makes you uncomfortable.
You can read about the study, and gain access to the survey here: Parental Mental Health Following a Traumatic Childbirth Survey.
Don’t forget to have your male or female partner also complete the survey. The childbirth experience affects the whole family.