Abstract
Introduction
A small placental surface at birth has been shown to be associated with the development of hypertension in later life. In this study we extend this observation by looking at the relationship between the number of placental cotyledons and blood pressure in childhood. Because the number of cotyledons is correlated with the surface area, we hypothesized that fewer cotyledons would be associated with higher blood pressure.
Methods
The Alspac study is a longitudinal study of 13,971 children born in Bristol. Their placentas were stored in formalin. We photographed the placentas of a sample of the children and related the number of maternal cotyledons to their blood pressure levels at age 9 years.
Results
Contrary to our hypothesis, a greater number of maternal cotyledons was associated with higher blood pressure. Among boys, a greater number of cotyledons was associated with higher systolic and diastolic pressure but not with higher pulse pressure. Diastolic pressure rose by 2.2 mmHg (95% CI 0.6 to 3.7, p =0.007) for every 10 additional cotyledons. Among girls, a greater number of cotyledons was associated with higher systolic pressure and pulse pressure but not with higher diastolic pressure. Pulse pressure rose by 2.7 mmHg (1.1 to 4.3, p<0.001) for every 10 additional cotyledons. These associations were little changed by adjustment for placental surface area.
Conclusion
Our study has shown that a large number of maternal cotyledons is associated with raised blood pressure in childhood. The associations differ in the two sexes.
Keywords: Alspac, maternal cotyledons, blood pressure
Introduction
People whose birthweights were towards the lower end of the normal range have higher blood pressures as children and adults than people whose birthweights were towards the upper end1, 2. This has led to the hypothesis that blood pressure levels are programmed in utero3. Programming is the phenomenon whereby minor variations in nutrient supply to different organs and systems during development permanently set the structure and function of the body. Birthweight is correlated with placental weight. Bigger babies generally have heavier placentas. Some studies have shown that hypertension in later life is related to low placental weight4, 5, while others have shown relationships with a high ratio of placental weight to birth weight5, 6. Which of these two relationships occurs depends on the mother’s height and socio-economic status5. The weight of the placenta, however, is a crude measure that does not distinguish the size of the surface from its thickness. Hypertension has been shown to be associated with a small placental surface at birth6. We now extend this observation by examining the association between the number of maternal cotyledons on the placental surface and blood pressure in childhood. Our hypothesis was that fewer cotyledons, associated with a smaller placental surface, would be associated with raised blood pressure.
The maternal cotyledons, or lobes, of the placenta are on the maternal side of the surface7. Each cotyledon is a perfusion chamber partly or wholly separated from adjacent chambers by a wall of connective tissue, which may be lined by trophoblast cells. One or more maternal spiral arteries jet blood into the chamber. The fetal cotyledons comprise one or more fetal villous trees, containing a fetal artery and a vein, that are suspended into the maternal cotyledon. The number of maternal cotyledons in the placenta is highly variable. What determines the number is unknown, though it may depend on events in early gestation.
Methods
ALSPAC is a longitudinal birth cohort study of the determinants of development, health, and disease during childhood and beyond8. Briefly, 14 541 pregnant women with an expected date of delivery between April 1991 and December 1992 were enrolled; 13 971 of their children formed the original cohort at 1 year of age. The pregnancies were selected irrespective of any complications that did or did not occur. The children have been repeatedly examined. The techniques used for blood pressure measurement have been described elsewhere9. In our study we have used blood pressure measurements and heart rates recorded at 9 years of age. After this age the adolescent growth spurt perturbs the tracking of blood pressure. The parents of each subject in the original study gave written informed consent. Ethical approval was granted by the ALSPAC Law and Ethics Committee and local research ethics committees.
At birth, the body size of the baby was measured. The length of gestation was estimated from the date of the mother’s last menstrual period. The placenta was stored in 10% formalin. In our study, we photographed a sample of 1746 stored placentas. The placentas, which all came from one maternity hospital, were taken in the order in which they were stored. They were selected without any knowledge of the measurements made at birth or during the follow-up of the children. The placentas were removed from their containers, rinsed, trimmed using a standard protocol and weighed. The maternal and fetal side of each placenta was photographed using a digital camera. Each photograph included a ruler to measure the length and breadth of the surface. The length was defined as the maximum diameter and the breadth was measured at right angles to the midpoint of the length. Assuming an elliptical surface, the placental area was defined as the product of the length and breadth, multiplied by π/4. The number of maternal cotyledons was counted. With the maternal side upwards an incision was made in each quadrant and the thickness measured through the incision.
Statistical methods
We used multiple linear regression analysis to examine the relationship between placental measurements and blood pressure, pulse pressure and heart rate. Blood pressures rose with increasing body weight and we adjusted for this and for age in our analyses. Because we found differences in the associations among boys and girls we present separate analyses for the two sexes.
Results
There were 910 subjects, 421 boys and 489 girls, for whom there were measurements of blood pressure at age 9 years. Table 1 shows the characteristics of the mothers, newborn babies and the nine year old children. Girls were smaller than boys for each measurement of body size at birth and for each placental measurement, except for the number of maternal cotyledons. The number of cotyledons ranged from 3 to 37. On average girls had 0.7 more cotyledons than boys (95% confidence interval 0.3 to 1.2, p=0.001).
Table 1. Characteristics of the study sample.
| Boys (n=421) | Girls (n=489) | |||||
|---|---|---|---|---|---|---|
|
|
||||||
| Mean | SD | Missing | Mean | SD | Missing | |
| Mother | ||||||
| Height (cm) | 164.5 | 6.5 | 21 | 164.5 | 7.0 | 19 |
| Weight before pregnancy (kg) | 62.1 | 10.5 | 29 | 61.5 | 10.3 | 36 |
| Body mass index (kg/m2) | 23.0 | 3.8 | 30 | 22.7 | 3.8 | 40 |
| Age (years) | 29.4 | 4.5 | 0 | 28.8 | 4.3 | 1 |
| Primiparous (%) | 45.9 | - | 11 | 46.4 | - | 13 |
| Newborn | ||||||
| Weight (g) | 3529 | 518 | 7 | 3422 | 445 | 4 |
| Length (cm) | 51.3 | 2.1 | 42 | 50.4 | 1.9 | 42 |
| Head circumference (cm) | 35.3 | 1.3 | 39 | 34.6 | 1.1 | 40 |
| Gestation (weeks) | 39.5 | 1.6 | 0 | 39.7 | 1.4 | 1 |
| Placenta | ||||||
| Weight (g) | 553 | 112 | 1 | 546 | 99 | 0 |
| Length (cm) | 20.4 | 2.4 | 0 | 20.2 | 2.2 | 0 |
| Width (cm) | 17.8 | 2.0 | 0 | 17.7 | 1.8 | 0 |
| Area (cm2) | 288 | 60 | 0 | 282 | 52 | 0 |
| Thickness (cm) | 2.8 | 0.5 | 0 | 2.8 | 0.5 | 0 |
| Number of cotyledons | 12.7 | 4.5 | 60 | 13.6 | 4.4 | 39 |
| Child at age 9 years | ||||||
| Age (years) | 9.8 | 0.3 | 0 | 9.8 | 0.3 | 0 |
| Height (cm) | 139.8 | 6.3 | 4 | 139.4 | 6.3 | 11 |
| Weight (kg) | 34.4 | 7.0 | 0 | 35.3 | 7.6 | 1 |
| Body mass index (kg/m2) | 17.5 | 2.7 | 4 | 18.1 | 3.0 | 11 |
| Systolic blood pressure (mm Hg) | 102.5 | 9.2 | 0 | 101.9 | 9.1 | 0 |
| Diastolic blood pressure (mm Hg) | 57.0 | 6.9 | 0 | 57.7 | 6.0 | 0 |
| Pulse pressure (mm Hg) | 45.5 | 7.6 | 0 | 44.3 | 7.9 | 0 |
| Heart rate (beats/min) | 76.6 | 10.1 | 0 | 80.4 | 10.7 | 0 |
Both systolic and diastolic pressure fell with increasing birth weight. Among boys mean systolic pressure fell by 2.0 mm (95% CI 0.5 to 3.5, p=0.008) per kg increase in birth weight, while diastolic pressure fell by 1.0 mm Hg (−0.2 to 2.2, p=0.1). The corresponding figures for girls were 3.0 (1.3 to 4.6, p<0.001) and 1.7 (0.5 to 2.9, p=0.006). These values were little changed by adjustment for gestational age at delivery, which was not associated with blood pressure.
Number of cotyledons
Heavier birthweight was associated with a greater number of cotyledons. A one kg increase in birth weight was associated with a 1.3 (95% CI 0.6 to 1.9, p<0.001) increase in the mean number of cotyledons among boys, and a 0.8 (0.1 to 1.5, p=0.03) increase among girls. The number of cotyledons was positively related to the surface area. Every 100 cm2 increase in area was associated with a 1.7 (1.1 to 2.2, p<0.001) increase in the average number of cotyledons among boys, and a 2.2 (1.6 to 2.8, p<0.001) increase among girls. The number of cotyledons was not independently related to any other placental measurement.
Table 2 shows the relationship between placental size and blood pressure and heart rate. Among boys a greater number of cotyledons was associated with higher systolic and diastolic pressure, but not with higher pulse pressure or heart rate. Among girls a greater number of cotyledons was associated with higher systolic and pulse pressure, and higher heart rate, but not with higher diastolic pressure. The trends with the number of cotyledons in boys and girls are shown in table 3. The trends in diastolic pressure and pulse pressure were statistically significantly different in the two sexes (p for interaction = 0.002 and 0.02).
Table 2. Correlation coefficients of placental size with blood pressure and heart rate.
| Placental measurement |
Systolic Blood Pressure | Diastolic Blood Pressure | Pulse Pressure | Heart Rate | ||||
|---|---|---|---|---|---|---|---|---|
|
| ||||||||
| Correlation | p-value | Correlation | p-value | Correlation | p-value | Correlation | p-value | |
| BOYS | ||||||||
| No. cotyledons | 0.12 | 0.03 | 0.14 | 0.007 | −0.01 | 0.9 | −0.02 | 0.7 |
| Length | −0.10 | 0.05 | −0.05 | 0.3 | −0.06 | 0.2 | −0.01 | 0.8 |
| Width | −0.03 | 0.5 | −0.03 | 0.6 | −0.01 | 0.9 | 0.02 | 0.6 |
| Weight | −0.04 | 0.4 | −0.10 | 0.05 | 0.04 | 0.4 | 0.05 | 0.3 |
| GIRLS | ||||||||
| No. cotyledons | 0.09 | 0.05 | −0.08 | 0.1 | 0.16 | <0.001 | 0.09 | 0.05 |
| Length | −0.05 | 0.2 | −0.11 | 0.01 | 0.03 | 0.5 | −0.02 | 0.6 |
| Width | −0.05 | 0.2 | −0.11 | 0.02 | 0.03 | 0.6 | 0.01 | 0.9 |
| Weight | −0.04 | 0.4 | −0.04 | 0.4 | −0.01 | 0.9 | 0.00 | 0.9 |
Table 3. Mean blood pressures and heart rates according to number of maternal cotyledons.
| No. cotyledons | Systolic blood pressure (mm Hg) |
Diastolic blood pressure (mm Hg) |
Pulse pressure (mm Hg) |
Heart Rate (beats/min) |
No. children |
|---|---|---|---|---|---|
| BOYS | |||||
| −8 | 101.3 | 55.7 | 45.6 | 77.8 | 65 |
| 9-11 | 101.8 | 56.5 | 45.3 | 75.9 | 94 |
| 12-14 | 103.6 | 57.5 | 46.1 | 77.0 | 79 |
| 15-17 | 103.6 | 57.4 | 46.2 | 77.1 | 69 |
| 18+ | 103.2 | 58.8 | 44.3 | 76.3 | 54 |
| p for trend | 0.05 | 0.007 | 0.7 | 0.7 | |
| mm Hg per 10 cotyledons | 1.9 (0.0 to 3.7) | 2.2 (0.6 to 3.7) | −0.3 (−2.0 to 1.3) | −0.5 (−2.9 to 1.9) | |
| GIRLS | |||||
| −8 | 100.2 | 58.7 | 41.4 | 76.8 | 55 |
| 9-11 | 101.5 | 57.7 | 43.8 | 81.6 | 86 |
| 12-14 | 101.9 | 57.4 | 44.5 | 80.0 | 128 |
| 15-17 | 103.3 | 58.2 | 45.1 | 82.3 | 103 |
| 18+ | 102.2 | 56.3 | 45.9 | 80.3 | 77 |
| p for trend | 0.05 | 0.1 | <0.001 | 0.05 | |
| mm Hg per 10 cotyledons | 1.7 (0.0 to 3.4) | −1.0 (−2.3 to 0.2) | 2.7 (1.1 to 4.3) | 2.2 (−0.1 to 4.5) |
Placental Surface
There were different associations between the length and breadth of the placental surface and later blood pressure in the two sexes (Table 2). Among boys greater length of the placental surface was associated with lower systolic pressure. Among girls, greater length, breadth and area of the placental surface were associated with lower diastolic pressure. In simultaneous regressions with length, breadth or area, the effects of number of cotyledons on blood pressure were little changed. The thickness of the placenta was not associated with blood pressure in either sex.
Discussion
Contrary to our hypothesis, we found that a greater number of maternal cotyledons at birth was associated with higher blood pressure at 9 years of age. The associations differed in the two sexes. Among boys, a greater number of cotyledons was associated with higher systolic and diastolic pressure but not with higher pulse pressure. Among girls, a greater number of cotyledons was associated with higher systolic pressure and pulse pressure but not with higher diastolic pressure.
As expected from many other studies of blood pressure in childhood1, we found that lower birthweight was associated with higher systolic and diastolic blood pressure. Although placentas with a larger surface area had more cotyledons, raised blood pressure was associated with a smaller length, breadth or area of the placental surface. This is consistent with findings linking a small placental surface to later hypertension6. The association between cotyledon number and blood pressure was little changed by allowing for the size of the surface. This suggests that the processes that link greater cotyledon number to raised blood pressure differ from those that link small surface area to raised blood pressure.
Our thesis is that cotyledon number is linked to settings in the fetal autonomic nervous system while area is linked to nutrient supply. A placenta with a large number of cotyledons will have more walls of connective tissue separating the cotyledons. This tissue is not involved in nutrient exchange but will have a continuing demand for nutrients in order to maintain itself. Notwithstanding this we found that a larger number of cotyledons was related to heavier birthweight, indicating that more cotyledons does not reduce nutrient supply to the fetus, but enhances it. We speculate that, for whatever reason, this is associated with increased stress to the fetus, leading to altered settings of the autonomic nervous system that increase vascular tone. Persistence of increased vascular tone after birth leads to raised blood pressure. Another possible explanation for the link between cotyledon number and later blood pressure is that it reflects an altered structure of the villous vascular tree. Boys and girls grow differently in utero10, and there are sex differences in the associations between cotyledon number and blood pressure.
Systolic blood pressure is determined by stroke volume, vascular stiffness and peripheral resistance. Diastolic pressure is determined by peripheral resistance and heart rate. Pulse pressure is determined by stroke volume and peripheral resistance. A larger number of cotyledons among boys predicted raised systolic and diastolic pressure and we infer that boys with a larger number of cotyledons have increased peripheral resistance as a result of increased sympathetic tone. A larger number of placental cotyledons among girls predicted raised systolic and pulse pressure but not diastolic pressure. More cotyledons in girls was also associated with higher heart rate. The association with raised pulse pressure cannot therefore be attributed to an association with reduced heart rate. We infer that girls with larger number of cotyledons have an imbalance in the autonomic nervous system such that the balance of sympathetic to parasympathetic tone is increased. There may, however, be other explanations.
Limitations of the Study
Our study is based on one population in Southern England and the findings may not be generalizable to other places. We are therefore seeking to replicate them in another country. The number of cotyledons in our sample ranged from 3 to 37, which is wider ranges previously cited11. Although blood pressure levels are known to track through childhood into adult life, the long term significance of differences in childhood blood pressure will need to be established through the continued follow-up of the ALSPAC cohort.
Conclusion
Our study has shown, for the first time, that a large number of maternal cotyledons is associated with a raised blood pressure in childhood. The associations are different in the two sexes. We speculate that they reflect altered settings of the autonomic nervous system.
Acknowledgements
We are extremely grateful to all the families who took part in this study, the midwives for their help in recruiting them, and the whole ALSPAC team, which includes interviewers, computer and laboratory technicians, clerical workers, research scientists, volunteers, managers, receptionists and nurses. The UK Medical Research Council (Grant ref: 74882) the Wellcome Trust (Grant ref: 076467) and the University of Bristol provide core support for ALSPAC. This publication is the work of the authors and D.J.P.Barker and C.Osmond will serve as guarantors for the contents of this paper. This research was specifically funded by the Medical School and the MRC Lifecourse Epidemiology Unit at the University of Southampton.
References
- 1.Huxley RR, Shiell AW, Law CM. The role of size at birth and postnatal catch-up growth in determining systolic blood pressure: a systematic review of the literature. J Hypertens. 2000;18:815–31. doi: 10.1097/00004872-200018070-00002. [DOI] [PubMed] [Google Scholar]
- 2.Barker DJ, Forsén T, Eriksson JG, Osmond C. Growth and living conditions in childhood and hypertension in adult life: a longitudinal study. J Hypertens. 2002;20:1951–6. doi: 10.1097/00004872-200210000-00013. [DOI] [PubMed] [Google Scholar]
- 3.Barker DJP. ‘Fetal origins of coronary heart disease. Brit Med J. 1995;311:171–174. doi: 10.1136/bmj.311.6998.171. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Eriksson J, Forsén T, Tuomilehto J, Osmond C, Barker D. Fetal and childhood growth and hypertension in adult life. Hypertension. 2000;36:790–4. doi: 10.1161/01.hyp.36.5.790. [DOI] [PubMed] [Google Scholar]
- 5.Barker DJ, Thornburg KL, Osmond C, Kajantie E, Eriksson JG. The surface area of the placenta and hypertension in the offspring in later life. Int J Dev Biol. 2010;54:525–30. doi: 10.1387/ijdb.082760db. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Barker DJP, Bull AR, Osmond C, Simmonds SJ. Fetal and placental size and risk of hypertension in adult life’. British Medical Journal. 1990;301:259–262. doi: 10.1136/bmj.301.6746.259. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Benirschke K, Kaufman P, Baergen R. Pathology of the Human Placenta. 5th ed Springer, New York: 2006. [Google Scholar]
- 8.Sherriff A, Emond A, Bell JC, Golding J. Should infants be screened for anaemia? A prospective study investigating the relation between haemoglobin at 8, 12, and 18 months and development at 18 months. Arch Dis Child. 2001;84:480–5. doi: 10.1136/adc.84.6.480. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Jones A, Charakida M, Falaschetti E, Hingorani AD, Finer N, Masi S, et al. Adipose and height growth through childhood and blood pressure status in a large prospective cohort study. Hypertension. 2012;59:919–925. doi: 10.1161/HYPERTENSIONAHA.111.187716. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Eriksson JG, Kajantie E, Osmond C, Thornburg K, Barker DJ. Boys live dangerously in the womb. Am J Hum Biol. 2010;22:330–5. doi: 10.1002/ajhb.20995. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Hamilton WJ, Mossman HW. Hamilton, Boyd and Mossman’s Human Embryology. 4th ed Heffer; Cambridge: 1972. [Google Scholar]